A novel approach that allows control of flow in microfluidic channels with unsurpassed performance using light is described. Valve structures have been created using photoresponsive hydrogels based on spiropyran-functionalised pNIPAAm hydrogels photopolymerised around pillar structures within the channels. Valve actuation is controlled from outside the fluidic system using externally located LEDs. Highly precise and accurate flow rates can be selected by passing real-time flow rate measurements into a PID algorithm. The optimised algorithm also minimises overshoot of the selected flow rate, eliminates flow rate drift, and improves the system response time. In addition to the dramatic improvements in flow rate control, the set up enables the polymer actuation behaviour to be rapidly characterised. The power supply to the LED also provides a useful system diagnostic for monitoring the performance of the valve over time. For example, degradation in the valve actuation due to photodegradation will manifest as an increasing power requirement over time, enabling predictive failure thresholds to be established for particular actuator designs and polymer compositions.

The ability to non-invasively monitor sodium levels in sweat is of significant importance. Sodium is one of the preferred markers to diagnose and track the progression of cystic fibrosis, and knowledge of sodium levels could potentially enable personalised hydration strategies to be implemented for athletes or people working under severe environmental conditions. Herein we present a novel approach for the realisation of disposable potentiometric strips that allow for real-time monitoring of sodium in sweat. Our platform consists of a Solid-Contact Ion-Selective Electrode (SC-ISE) for Na+ detection and of a liquid-junction-free Reference Electrode (RE), combined together on a dual screen-printed substrate. Different poly-3,4-ethylenedioxythiophene (PEDOT) based films were tested as solid-contact, showing a significant impact on sensor characteristics such as sensitivity (i.e. differing from sub-Nernstian to Nernstian), dynamic range (i.e. 10−5 to 10−2.5 or 10−5 to 10−1aNa+), and especially within-batch reproducibility. The SC-ISE/RE combination was integrated into a microfluidic chip that was tested and optimised via on-bench trials. The Potentiometric Microfluidic Chip (PotMicroChip) was then connected to a wireless electronic platform to realise a wearable device whose performance was assessed during real-time stationary cycling sessions.

Reversible light-responsive hydrogel valves with response characteristics compatible for microfluidics have been obtained by optimization of molecular design of spiropyran photoswitches and gel composition. Self-protonating gel formulations were exploited, wherein acrylic acid was copolymerized in the hydrogel network as an internal proton donor, to achieve a swollen state of the hydrogel in water at neutral pH. Light-responsive properties were endowed upon the hydrogels by copolymerization of spiropyran chromophores, using electron withdrawing and donating groups to tune the gel-swelling and shrinkage behavior. In all cases, the shrinkage was determined by the water diffusion rate, while for the swelling the isomerization kinetics is the rate-determining step. For one hydrogel, reversible and reproducible volume changes were observed. Finally, gel-valves integrated within microfluidic channels were fabricated, allowing reversible and repeatable operation, with opening and closing of the valve in minutes.

Gold nanoparticles (Au-NPs) functionalised with lipoic acid (LAc) or lipoic amide (LAm) have been investigated as solid-contact in disposable lead and sodium selective electrodes. In Pb2+ and Na+ solutions, electrodes incorporating LAm Au-NPs displayed super-Nernstian and Nernstian behaviour whereas electrodes incorporating LAc Au-NPs showed Nernstian and sub-Nernstian responses. During calibration, the response of solid-contact ion-selective electrodes (SC-ISEs) was found to depend on the particular ion-analyte/Au-NP ligand pair, suggesting that the ligand decorating the NPs has an important role in regulating the overall electrode response function. Insights into the underlying trans-membrane and the Au-NPs/membrane interface processes have been obtained by means of electrochemical impedance spectroscopy.

Progress towards the development of a miniaturised microfluidic instrument for the direct measurement of nitrate in natural waters and wastewater using chromotropic acid is presented. For the first time, the chromotropic method for nitrate analysis has been transferred to a microfluidic chip configuration that can withstand the extremely acidic nature of the reagent within a field deployable platform. This simple method employs one reagent mixed in a 1:1 ratio with the sample to produce a yellow colour absorbing strongly at 430 nm. A stopped flow approach is used which, together with the very rapid kinetics and simple reagent stream, enables an uncomplicated microfluidic design and field deployable platform with a sample throughput of 9 samples per h, limits of detection of 0.70 mg L−1 NO3− and 0.31 mg L−1 NO3− for seawater samples, with a dynamic linear range from 0–80 mg L−1 NO3− and long-term reagent stability of up to 6 months. Validation was achieved by analysing split water samples by the analyser and ion chromatography, resulting in an excellent correlation co-efficient of 0.9969. The fully integrated sensing platform consists of a sample inlet with filter, storage units for chromotropic reagent and standards for self-calibration, pumping system which controls the transport and mixing of the sample, a microfluidic mixing and detection chip, and waste storage, all contained within a ruggedized, waterproof housing. The optical detection system consists of a LED light source with a photodiode detector, which enables sensitive detection of the coloured complex formed. The low cost of the platform coupled with integrated wireless communication makes it an ideal platform for in situ environmental monitoring.

Here we describe, for the first time, intriguing solvato-morphological control of spiropyran-based microcrystalline structures. These microstructures exhibit reversible photoisomerization upon light irradiation (UV/Vis) in the solid-state. Finally, light-guided aggregation of these microstructures at the liquid/air interface is also demonstrated.

The physicochemical properties of free-standing cross-linked poly(N-isopropylacrylamide) (pNIPAAM) gels, generated in the presence of the Ionic liquids (ILs), 1-ethyl-3-methylimidazolium [C2mIm]+ salts of ethylsulfate [EtSO4]−, dicyanamide [DCA]−, bis(trifluoromethylsulfonyl)imide [NTf2]−, and trihexyltetradecylphosphonium dicyanamide ([P6,6,6,14][DCA]) are described. The Lower Critical Solution Temperature (LCST) of the resulting ionogel was found to vary between 24–31 °C. The behaviour of swelling is found to be as high as 31.55% (±0.47, n = 3) from the initial dehydrated state, while 28.04% (±0.42, n = 3) shrinking from the hydrated swollen state is observed. For ionogels based on the [DCA]− anion an unexpected complete loss of the shrinking behaviour occurs, attributed to water interactions with the nitrile group of the [DCA]− anion. Scanning Electron Microscopy also reveals distinct morphological changes, for example [C2mIm][EtSO4] displays a highly porous, nodule type morphology, efficiently pre-disposed for water uptake.

The ability to switch the physico-chemical properties of conducting polymers opens up new possibilities for a range of applications. Appropriately functionalised materials can provide routes to multi-modal switching, for example, in response to light and/or electrochemical stimuli. This capability is important in the field of bionics wherein remote and temporal control of the properties of materials is becoming attractive. The ability to actuate a film via photonic stimuli is particularly interesting as it facilitates the modulation of interactions between host binding sites and potential guest molecules. In this work, we studied two different poly-terthiophenes: one was functionalised with a spiropyran photoswitch (pTTh-SP) and the second with a non-photoswitchable methyl acetate moiety (pTTh-MA). These substrates were exposed to several cycles of illumination with light of different wavelengths and the resulting effect studied with UV-vis spectroscopy, contact angle and atomic force microscopy (AFM). The AFM tips were chemically activated with fibronectin (FN) and the adhesion force of the protein to the polymeric surface was measured. The pTTh-MA (no SP incorporated) showed a slightly higher average maximum adhesion (0.96 ± 0.14 nN) than the modified pTTh-SP surface (0.77 ± 0.08 nN), but after exposure of the pTTh-SP polymer to UV, the average maximum adhesion of the pTTh-MC (merocyanine form) was significantly smaller (0.49 ± 0.06 nN) than both the pTTh-MA and pTTh-SP. In addition, the tip-sample separation distances of the adhesive interactions are indicative of the FN interaction occurring over a distance more closely related to the average dimensions of its compact conformation. The results suggest that surface energy and hydrophobic forces are predominant in determining the protein adhesion to the films studied and that this effect can be photonically tuned. By extension, this further implies that it should be possible to obtain a degree of spatial and temporal control of the surface binding behaviour of certain proteins with these functionalised surfaces through photo-activation/deactivation, which, in principle, should facilitate patterned growth behaviour (e.g. using masks or directional illumination) or photocontrol of protein uptake and release.

A new series of LCST ILs have been copolymerised with crosslinkers of varying length to afford the first ever thermoresponsive poly(ionic liquid)-based hydrogels. These hydrogels exhibit surprisingly broad LCST and volume transition behaviour compared to standard thermoresponsive gels and linear ILs.

A crosslinked poly(N-isopropylacrylamide) ionogel encapsulating an ionic liquid exhibits improved transmittance properties, enhanced water uptake/release, greater thermal actuation behaviour and distinct solvatomorphology over its hydrogel equivalent. It was also found that the rate of release of fluorescein pre-loaded into membranes was considerably enhanced for ionogels compared to equivalent hydrogels, and could be triggered through changes in pH and temperature.

This work describes the synthesis and characteristics of a novel electrochromic ionic liquid (IL) based on a phosphonium core tethered to a viologen moiety. When integrated into a solid-state electrochromic platform, the viologen modified IL behaved as both the electrolyte and the electrochromic material. Platform fabrication was achieved through in situ photo-polymerization and encapsulation of this novel IL within a hybrid sol–gel. Important parameters of the platform performance, including its coloration efficiency, switching kinetics, and optical properties were characterised using UV–vis spectroscopy and cyclic voltammetry in tandem. The electrochromic platform exhibits a coloration efficiency of 10.72 cm2 C–1 and a varied optical output as a function of the incident current. Despite the rather viscous nature of the material, the platform exhibited approximately 2 orders of magnitude faster switching kinetics (221 s to reach 95 % absorbance) when compared to previously reported electrochromic ILs (18 000 s).Keywords: electrochromic devices; electrochromic materials; Ionic liquids; solid-state electrolytes; viologens;

Up to now, photoresponsive hydrogels and ionogels based on poly(N-isopropylacrylamide) copolymerised with pendant spiropyran groups require exposure to external acidic solution (usually milimolar HCl) to generate the swollen gel prior to photo-triggered contraction. This serious functional limitation has been solved by copolymerising acrylic acid into the gel matrix, to provide an internal source of protons. Due to the relative pKa values of acrylic acid and the spiropyran and merocyanine isomers, the protonation and deprotonation occurs internally within the gel and there is no need for an external source of protons. Furthermore, the shrinking–expansion cycles of these gels in deionised water are repeatable, as protonation throughout the gel does not rely on movement of protons from an external acidic solution into the bulk gel. In contrast to previous formulations, these gels do not show degradation of their photo-induced shrinking ability after multiple washings in deionised water and repeated switching over a 2 month period.

This work describes the first use of a direct nitrate analyser using chromotropic acid. A simplified chromotropic acid method eliminating several steps previously associated with this method is employed in the platform. In a sulphuric acid medium, chromotropic acid reacts with nitrate ions and produces a characteristic yellow colour associated with an absorbance band in the visible region (λmax = 430 nm). The modified method allows for nitrate determination over the linear range 0.9–80 mg L−1 nitrate with a limit of detection of 0.73 μg L−1 nitrate. Validation was achieved by analysing water samples from various sources including groundwater, trade effluent and drinking water by the modified method and by ion chromatography. The method was implemented on a flow analysis platform incorporating a paired emitter–detector diode (PEDD) as the optical detector. An excellent correlation coefficient of 0.993 was obtained between the modified method and ion chromatography. The modified chromotropic acid method represents a rapid, simple, low cost technique for the direct determination of nitrate in water.

Solid-contact Pb2+-selective-electrodes and solid contact reference electrodes suitable for use as disposable sensing devices for environmental monitoring of lead have been prepared on screen-printed substrates. Accurate control over the fabrication procedures leads to excellent reproducibility of their calibration characteristics such as slope, offset and limit of detection. In particular, the limit of detection in the nanomolar range opens the possibility of their use for trace analysis of Pb2+ in environmental water samples. Significantly, the potentiometric measurements correlate well with data determined using inductively coupled plasma mass spectrometry (ICP-MS) in a number of real samples taken from local rivers. Ways in which these sensors might be employed in autonomous platforms for monitoring water quality in situ are discussed. The possibility of including arrays of virtually identical sensors is highlighted as a possible route to achieve long-term deployments.

The photochemistry and thermodynamics of two terthiophene (TTh) derivatives bearing benzospiropyran (BSP) moieties, 1-(3,3″-dimethylindoline-6′-nitrobenzospiropyranyl)-2-ethyl 4,4″-didecyloxy-2,2′:5′,2″-terthiophene-3′-acetate (BSP-2) and 1-(3,3″-dimethylindoline-6′-nitrobenzospiropyranyl)-2-ethyl 4,4″-didecyloxy-2,2′:5′,2″-terthiophene-3′-carboxylate (BSP-3), differing only by a single methylene spacer unit, have been studied. The kinetics of photogeneration of the equivalent merocyanine (MC) isomers (MC-2 and MC-3, respectively), the isomerisation properties of MC-2 and MC-3, and the thermodynamic parameters have been studied in acetonitrile, and compared to the parent, non-TTh-functionalised, benzospiropyran derivative, BSP-1. Despite the close structural similarity of BSP-2 and BSP-3, their physicochemical properties were found to differ significantly; examples include activation energies (Ea(MC-2) = 75.05 kJ mol−1, Ea(MC-3) = 100.39 kJ mol−1) and entropies of activation (ΔS‡MC-2 = 43.38 J K−1 mol−1, ΔS‡MC-3 = 37.78 J K−1 mol−1) for the thermal relaxation from MC to BSP, with the MC-3 value much closer to the unmodified MC-1 value (46.48 J K−1 mol−1) for this latter quantity. The thermal relaxation kinetics and solvatochromic behaviour of the derivatives in a range of solvents of differing polarity (ethanol, dichloromethane, acetone, toluene and diethyl ether) are also presented. Differences in the estimated values of these thermodynamic and kinetic parameters are discussed with reference to the molecular structure of the derivatives.

This work details the use of a 2-component optode membrane which is capable of generating three distinct colours in the presence of Cu2+ and Co2+ ions. It has been found that the ionic liquid (IL) trihexyltetradecylphosphonium dicyanamide [P6,6,6,14][DCA] can act as plasticizer, ligand and transducer dye when used in poly(vinyl chloride) (PVC) membranes, which significantly simplifies the optode membrane cocktail. Upon exposure to an aqueous Cu2+ solution, a yellow colour is generated within the membrane, while exposure to an aqueous Co2+ solution generates a blue colour. Exposure to a solution containing both ions produces a green colour. Vibrational spectroscopy has been used to investigate the molecular basis of the IL–metal ion the binding mechanism. Analytical characteristics of the membranes including the effect of interfering ions, binding constants and the limit of detection for both ions have been estimated. Finally the case of simultaneous dual-analyte recognition is presented based on two distinct absorption maxima.

With recent improvements in wireless sensor network hardware there has been a concurrent push to develop sensors that are suitable in terms of price and performance. In this paper a low-cost gas sensor is detailed, and significant improvements in sensor characteristics have been achieved compared to previously published results. A chemical sensor is presented based on the use of low-cost LEDs as both the light source and photodetector, coupled with a sensor slide coated with a pH sensitive colorimetric dye to create a simple gas sensor. Similar setups have been successfully used to detect both acetic acid and ammonia. The goal of this work was to optimise the system performance by integration of the sensing technique into a purposely deigned flowcell platform that holds the colorimetric slide and optical detector in position. The reproducibility of the sensor has been improved through this arrangement and careful control of deposited film thickness. The enhanced reproducibility between sensors opens the potential of calibration-free measurement, in that calibration of one sensor can be used to model the characteristics of all sensors in a particular batch.

We present the fabrication, characterisation and performance of four novel ionic liquid polymer gels (ionogels) as photo-actuated valves incorporated into micro-fluidic manifolds. The ionogels incorporate benzospiropyran units and phosphonium-based ionic liquids. Each ionogel is photo-polymerised in situ in the channels of a poly(methyl methacrylate) micro-fluidic device, generating a manifold incorporating four different micro-valves. The valves are actuated by simply applying localised white light irradiation, meaning that no physical contact between the actuation impulse (light) and the valve structure is required. Through variation of the composition of the ionogels, each of the micro-valves can be tuned to open at different times under similar illumination conditions. Therefore, flows through the manifold can be independently controlled by a single light source. At present, the contraction process to open the channel is relatively rapid (seconds) while the recovery (expansion) process to re-close the channel is relatively slow (minutes), meaning that the valve, in its current form, is better suited for single-actuation events.

We investigate the physicochemical properties of a novel imidazolium benzospiropyran derivative, SPIm, in imidazolium based ionic liquids (ILs). SPIm was prepared through alkylation of an imidazole to the photoswitchable compound and this derivative was characterised in imidazolium based ILs with increasing chain length to examine the stability of its merocyanine (MC) and spiropyran (SP) forms and compared to standard spiropyran, BSP. The rate of thermal relaxation of the new derivative is found to be about ten times faster than that of BSP as reflected in rates of 13.9 × 10−3 s−1 and 1.0 × 10−3 s−1 for SPIm and BSP, respectively, in [C6mIm][NTf2]. Since ILs are believed to form nano-structured domains it is proposed that the covalent attachment of the imidazolium side group of SPIm fully integrates the photoswitchable moiety into the non-polar region through side-chain association. In contrast, unbound BSP is relatively free to migrate between both polar and non-polar regions and the MC form is more readily stabilised by the IL charge via through space interactions and spontaneous movement to charged nano-domains leading to enhancement of the MC lifetime. At higher concentrations, rheological and transport properties were investigated to determine the impact of covalent attachment of the BSP fragment to an imidazolium cation on the ionic liquid structure. Ionic conductivity was found to decrease by up to 23% for SPIm with effects increasing with cation side-chain length. Unlike BSP, the photoswitching of the SPIm did not affect conductivity or viscosity values. This may indicate that the mobility of the photoswitchable compound and the resulting disruption of such movement may be critical to the control of this physical property.

A procedure for the development of a pen-like, multi-electrode potentiometric sensing platform is described. The platform comprises a seven-in-one electrode incorporating all-solid-state ion-selective and reference electrodes based on the conductive polymer (poly(3,4-ethylenedioxythiophene) (PEDOT)) as an intermediate layer between the contacts and ion-selective membranes. The ion-selective electrodes are based on traditional, ionophore-based membranes, while the reference electrode is based on a polymer membrane doped with the lipophilic salt tetrabutyl ammonium tetrabutyl borate (TBA-TBB). The electrodes, controlled with a multichannel detector system, were used for simultaneous determination of the concentration of Pb2+ and pH in environmental water samples. The results obtained using pH-selective electrodes were compared with data obtained using a conventional pH meter and the average percent difference was 0.3%. Furthermore, the sensing system was successfully used for lead-speciation analysis in environmental water samples.

The cost of monitoring and detecting pollutants in natural waters is of major concern. Current and forthcoming bodies of legislation will continue to drive demand for spatial and selective monitoring of our environment, as the focus increasingly moves towards effective enforcement of legislation through detection of harmful events, and unambiguous identification of perpetrators. However, these monitoring demands are not being met due to the infrastructure and maintenance costs of conventional sensing models. Advanced autonomous platforms capable of performing complex analytical measurements at remote locations still require individual power, wireless communication, processor and electronic transducer units, along with regular maintenance visits. Hence the cost base for these systems is prohibitively high, and the spatial density and frequency of measurements are insufficient to meet requirements. In this paper, we present a more cost effective approach for water quality monitoring using a low cost mobile sensing/communications platform together with very low cost stand-alone ‘satellite’ indicator stations that have an integrated colorimetric sensing material. The mobile platform is equipped with a wireless video camera that is used to interrogate each station to harvest information about the water quality. In simulation experiments, the first cycle of measurements is carried out to identify a ‘normal’ condition followed by a second cycle during which the platform successfully detected and communicated the presence of a chemical contaminant that had been localised at one of the satellite stations.

In this paper we consider the critical issues inhibiting the widespread deployment of bio/chemo-sensors in wireless sensor networks. Primary among these is the problem of performing calibration at remote locations, and the consequent need for integrated fluidic systems for performing tasks like sampling, calibration and detection. Our conclusion is that low-cost, bio/chemo-sensing platforms that provide reliable information over long periods of use will only be realised through the use of microfluidic platforms that are much more biomimetic in nature than technologies employed in current devices. Central to driving down costs will be the development of fluidic platforms with integrated soft polymer actuators that will replace existing pumps and valves. A particularly attractive approach is to employ photo-controlled polymer actuators, wherein the status of the material can be effectively switched using light, as this allows physical separation of the control layer from the fluidic platform layer in a planar system. This, in principle, should greatly simplify manufacturing and therefore drive down costs. In this paper, we describe a polymeric gel and a linear polymer modified with a photochromic moiety and show that it is possible to utilize photochromic molecules for performing sensing and actuating functions.

The combination of post-column derivatisation and visible detection are regularly employed in ion chromatography (IC) to detect poorly absorbing species. Although this mode is often highly sensitive, one disadvantage is the increase in repeating baseline artifacts associated with out-of-sync pumping systems. The work presented here will demonstrate the use of a second generation design paired emitter–detector diode (PEDD-II) detection mode offering enhanced sensitivity to transition metals in IC by markedly reducing this problem and also by improving signal noise. First generation designs demonstrated the use of a single integrated PEDD detector cell as a simple, small (15 × 5 mm), highly sensitive, low cost photometric detector for the detection of metals in IC. The basic principle of this detection mode lies in the employment of two linear light emitting diodes (LEDs), one operating in normal mode as a light source and the other in reverse bias serving as a light detector. The second generation PEDD-II design showed increased sensitivity for Mn(II)- and Co(II)-2-(pyridylazo)resorcinol (PAR) complexes as a result of two simultaneously acquiring detection cells – one analytical PEDD cell and one reference PEDD cell. Therefore, the PEDD-II employs two wavelengths whereby one monitors the analyte reaction product and the second monitors a wavelength close to the isosbestic point. The optimum LED wavelength to be used for the analytical cell was investigated to maximise peak response. The fabrication process for both the analytical and reference PEDD cells was validated by determining the reproducibility of detectors within a batch. The reproducibility and sensitivity of the PEDD-II detector was then investigated using signals obtained from both intra- and inter-day chromatograms.

We have investigated the kinetic and thermodynamic parameters of thermal reversion of the spirooxazine 1,3,3-trimethyl-5′-(2-benzothiazolyl)-spiroindoline-2,3′-naphtho(2,1-b)(1,4) oxazine (SO) in molecular solvents and ionic liquids containing the [NTf2]− anion. ET(30) and Kamlet–Taft parameter studies were employed to interpret IL polarity and attempt to rationalise the kinetic–polarity relationship of MC→SO relaxation. The observed thermal relaxation of SO within ionic liquids was slower than that of molecular solvents with similar polarity, indicating a greater degree of interactions between the ionic liquid ions and the zwitterionic MC isomer, which led to increased lifetimes for the MC–ion complexes (19.6 s in acetonitrile and 90.9 s in [P6,6,6,14][NTf2]). Thermodynamic parameters of activation implied enhanced ordering of SO in ILs and its subsequent effect on thermal reversion of SO. Activation parameters found MC→SO relaxation was more temperature dependent in polar protic ILs such as [bmIm]+ than aprotic ILs such as [P6,6,6,14][NTf2] with ΔS‡ values of 40 J K mol−1 and −8.6 J K mol−1, respectively. Activation energies were generally higher in ILs (83.9–97 kJ mol−1) compared to molecular solvents (67.8–89.8 kJ mol−1), which was reflected in longer lifetimes of MC due to greater energy barriers of relaxation to SO. Pre-metathesis cleaning of precursor salts was found to be necessary in order to obtain spectroscopic grade ILs for physicochemical analysis using solvatochromic probe dyes. Inconsistencies in the polarity–kinetic relationship further reinforced the belief that measuring polarity of ILs may not be possible with solvatochromic probe dyes.

Carbon dioxide gas concentration determination using infrared gas sensors combined with Bayesian regularizing neural networks is presented in this work. Infrared sensor with a measuring range of 0–5% was used to measure carbon dioxide gas concentration within the range 0–15000 ppm. Neural networks were employed to fulfill the non-linear output of the sensor. The Bayesian strategy was used to regularize the training of the back propagation neural network with a Levenberg–Marquardt (LM) algorithm. By Bayesian regularization (BR), the design of the network was adaptively achieved according to the complexity of the application. Levenberg–Marquardt algorithm under Bayesian regularization has better generalization capability, and is more stable than the classical method. The results showed that the Bayesian regulating neural network was a powerful tool for dealing with the infrared gas sensor which has a large non-linear measuring range and provide precise determination of carbon dioxide gas concentration. In this example, the optimal architecture of the network was one neuron in the input and output layer and two neurons in the hidden layer. The network model gave a relationship coefficient of 0.9996 between targets and outputs. The prediction recoveries were within 99.9–100.0%.

Switchable materials have tremendous potential for application in sensor development that could be applied to many fields. We are focusing on emerging area of wireless sensor networks due to the potential impact of this concept in society. Spiropyran-based sensors are probably the most studied type of photoswitchable sensing devices. They suffer from many issues but photofatigue, insufficient selectivity and lack of sensitivity are probably the most important characteristics that hinder their wider application. Here, we are address these issues and demonstrate that covalent attachment of modified spiropyran into a polymeric film significantly reduces photodegradation. The observed signal loss after 12th cycle of switching between the spiropyran and merocyanine forms is only about 27% compared to the loss of 57% of the initial signal in an equivalent experiment based on non-immobilized spiropyran. This has enabled us to demonstrate at least five reversible cycles of detection of an ion of interest (in our case H+) with minimal signal loss. Furthermore, we demonstrate that the sensitivity can be increased by incorporation of additional binding groups in the parent spiropyran molecule. Using molecular modelling to calculate the relevant bond lengths as a measure of interaction between MC and H+, the calculated increase of H-bond strength is approximately an order of magnitude for a derivative containing a methoxy group incorporated in the o-position of the parent spiropyran in comparison to the equivalent unsubstituted phenol. This theoretical result was found to correspond very well with experimental observation. As a result, we have increased the sensitivity to H+ by approximately one order of magnitude.

In this paper, we show how through integrating the beneficial characteristics of micro-fluidic devices and spiropyrans dyes, a simple and very innovative chip configured as an on-line photonically controlled self-indicating system for metal ion accumulation and release can be realised. The micro-fluidic device consists of five independent 94 μm depth, 150 μm width channels fabricated in polydimethylsiloxane. The spiropyran 1′-(3-carboxypropyl)-3,3′-dimethyl-6-nitrospiro-1-benzopyran-2,2′-indoline is immobilised by physical adsorption into a polydimethylsiloxane matrix and covalently on the ozone plasma activated polydimethylsiloxane micro-channel walls. When the colourless, inactive, spiropyran coating absorbs UV light it switches to the highly coloured merocyanine form, which also has an active binding site for certain metal ions. Therefore metal ion uptake can be triggered using UV light and subsequently reversed on demand by shining white light on the coloured complex, which regenerates the inactive spiropyran form, and releases the metal ion. When stock solutions of several metal ions (Ca2+, Zn2+, Hg2+, Cu2+, Co2+) are pumped independently through the five channels, different optical responses were observed for each metal, and the platform can therefore be regarded as a micro-structured device for photo-controlled self-indicating metal ion complexation, accumulation and release.

Spiropyran derivatives have been immobilised on the surface of polystyrene microbeads using different immobilisation strategies. These functionalised polymeric beads can be reversibly switched between the colourless inactive spiropyran (SP) and highly coloured (purple) active merocyanine (MC) forms using low power light sources, such as light-emitting diodes (LEDs). A UV LED (375 nm) is used for the SP → MC conversion, and a white LED (430–760 nm) for the reverse MC → SP conversion. The photochromic behaviour of the coated beads has been characterised using different LEDs and reflection spectroscopy, employing optic fibres and an in-house-designed holder. Investigations into the metal-ion binding behaviour of the spiropyran-modified microbeads have shown that Cu2+ ions cause an appreciable colour and spectral change when brought into contact with the beads in the MC form, suggesting that a significant interaction is occurring. However, the Cu2+ ions can be completely expelled by photonic-conversion of the beads into the inactive SP form using a white LED. This sequence has been successfully repeated six times, suggesting that it is possible to cycle through activation of the functionalised beads from a non-binding to a binding form (SP → MC) using a UV LED, allow binding with Cu2+ ions to occur, and subsequently, expel the bound ions and regenerate the passive SP surface using a white LED. Other metals, such as calcium, do not cause any appreciable colour or spectral change over the same concentration range and in the presence of the same anion (final concentration 7.1 × 10−4 M nitrate salt in ethanol). The system is therefore self-indicating in terms of whether the active MC or inactive SP forms are present, and whether Cu2+ ions are bound to the MC form. In principle, therefore, these functionalised beads could form the basis of a photoswitchable stationary phase for metal ion binding and detection: irradiation of the stationary phase with UV LEDs causes retention of guest species due to the presence of the MC form, while subsequent exposure to white LEDs causes release of guest species into the mobile phase.

The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1′,3′,3′-trimethyl-6-nitrospiro-[1-benzopyran-2,2′-1H-indole] (BSP-NO2) were studied in ILs containing the anion [NTf2]− by UV-Vis absorption spectroscopy, ab initio molecular orbital theory and density functional theory (DFT) calculations. It was found that the kinetics and thermodynamics of the BSP-NO2 ↔ MC (merocyanine) equilibrium was sensitive to the nature of the cation. It was also observed that the imidazolium cation can form a through-space orbital interaction with the MC isomer, rather than a simple electrostatic interaction, thus preventing the MC conversion back to the BSP-NO2 isomer. The BSP-NO2 ↔ MC equilibrium thus serves as a model system for studying modes of interaction of the cations in ionic liquids.

Rapid developments in wireless communications are opening up opportunities for new ways to perform many types of analytical measurements that up to now have been restricted in scope due to the need to have access to centralised facilities. This paper will address both the potential for new applications and the challenges that currently inhibit more widespread integration of wireless communications with autonomous sensors and analytical devices. Key issues are identified and strategies for closer integration of analytical information and wireless communications systems discussed.

The use of a novel inexpensive photometric device, a paired emitter–detector diode (PEDD) has been applied to the colorimetric determination of phosphate using the malachite green spectrophotometric method. The novel miniaturized flow detector applied within this manifold is a highly sensitive, low cost, miniaturized light emitting diode (LED) based detector. The optical flow cell was constructed from two LEDs, whereby one is the light source and the second is the light detector, with the LED light source forward biased and the LED detector reversed biased. The photocurrent generated by the LED light source discharges the junction capacitance of the detector diode from 5 V (logic 1) to 1.7 V (logic 0) and the time taken for this process to occur is measured using a simple timer circuit.The malachite green (MG) method employed for phosphate determination is based on the formation of a green molybdophosphoric acid complex, the intensity of which is directly related to phosphate concentration. Optimum analytical parameters such as reaction kinetics, reagent to sample concentration ratio and emitter wavelength intensity were investigated for the spectrophotometric method. Linear calibration plots that obeyed the Beer–Lambert law were obtained for phosphate in the range of 0.02–2 μM. The dynamic range, sensitivity and limits of detection are reported.

There is much interest from the fisheries industry in developing rapid methods to evaluate real-time freshness of fish and seafood products. Emphasis is on the ones that would reflect and account for the products history and their storage conditions from “harvest-to-home”. The development of a “smart packaging” that monitors the microbial breakdown products in the headspace of packaged fish is described. When fish spoils it releases a variety of basic volatile amines which are detectable with appropriate pH indicating sensors. These are prepared by entrapping within a polymer matrix a pH sensitive dye that responds, through visible color changes to the spoilage volatile compounds that contribute to a quantity known as total volatile basic nitrogen (TVB-N). Laboratory trials on fresh fish filets showed that the sensor accurately tracks the increase in amines concentration in the package headspace. The response was also found to correlate to changing microbial populations (total viable count or TVC and Pseudomonas spp.). In addition, leaching of the dye was assessed over time to assess the suitability of the sensor formulation for food packaging application.

Urea–calix[4]arenes 1 and 2 were synthesised and incorporated into ISE membranes for assessment as sensors for inorganic anions in water. 1 revealed a strong response to all anions following the Hofmeister selectivity order. For ISEs of 2, the response to a portion of the anion series was suppressed, increasing the margin of selectivity of nitrate over chloride, a common interferant of nitrate in fresh and marine water samples. The performance of ISEs containing 2 was compared to commercially available alkylammonium nitrate ion-exchange salts used for nitrate sensing. Our ISEs performed favourably in terms of sensitivity, linear range and LOD with an improved selectivity coefficient over chloride of logKNO3−Cl−pot of −3.4, an order of magnitude better than commercially available nitrate ISEs. The pre-conditioning of ISEs in non-primary chloride salt was essential for obtaining these results.

Miniaturization of analytical devices through the advent of microfluidics and micro total analysis systems is an important step forward for applications such as medical diagnostics and environmental monitoring. The development of field-deployable instruments requires that the entire system, including all necessary peripheral components, be miniaturized and packaged in a portable device. A sensor for long-term monitoring of phosphate levels has been developed that incorporates sampling, reagent and waste storage, detection, and wireless communication into a complete, miniaturized system. The device employs a low-power detection and communication system, so the entire instrument can operate autonomously for 7 days on a single rechargeable, 12 V battery. In addition, integration of a wireless communication device allows the instrument to be controlled and results to be downloaded remotely. This autonomous system has a limit of detection of 0.3 mg/L and a linear dynamic range between 0 and 20 mg/L.

The concept of wireless sensor networks conjures up images of a world of ubiquitous sensing, but it requires a massive increase in the number of sensors available. Although there has been considerable activity in transducer-based 'sensor nets', there have been virtually no corresponding deployments of chemical- or biological-sensing networks — considerable advances in materials science are needed before these can be used on a large scale.

Photochromic spiropyran was covalently immobilized using alkyldiamine couplers with varying chain length (2, 4, 6, 8 methylene groups) onto an optically transparent polymeric surface. The relatively nonpolar and colourless spiropyran form can be reversibly switched to a charged, highly coloured, zwitterionic merocyanine isomer using UV and visible light, respectively. The merocyanine form complexes with metal ions, and this results in a shift in the absorbance spectrum, and a corresponding colour change is observed. Upon irradiation of the complex with visible light, the cation is released and the merocyanine (active) isomer reverts back to the spiropyran (passive) form. Optical switching and complexation behaviour of the polymer-bound spiropyran–merocyanine system is highly dependent on the tether length of the spiropyran from the polymeric surface. The effect is demonstrated using cobalt(II) chloride in solution.

A low-power, high sensitivity, very low-cost light emitting diode (LED)-based device for intensity-based light measurements is described. In this approach, a reverse-biased LED functioning as a photodiode is coupled with a second LED configured in conventional emission mode. A simple timer circuit measures how long (in microsecond) it takes for the photocurrent generated on the detector LED to discharge its capacitance from logic 1 (+5 V) to logic 0 (+1.7 V). The entire instrument provides an inherently digital output of light intensity measurements for a few cents. This light intensity dependent discharge process has been applied to measuring concentrations of coloured solutions and a mathematical model developed based on the Beer–Lambert Law.

A novel low power, low cost, highly sensitive, miniaturized light emitting diode (LED) based flow detector has been used as optical detector for the detection of sample components in high performance liquid chromatography (HPLC). This colorimetric detector employs two LEDs, one operating in normal mode as a light source and the other is reverse biased to work as a light detector. Instead of measuring the photocurrent directly, a simple timer circuit is used to measure the time taken for the photocurrent generated by the emitter LED (λmax 500 nm) to discharge the detector LED (λmax 621 nm) from 5 V (logic 1) to 1.7 V (logic 0) to give digital output directly without using an A/D converter. Employing a post-column reagent method, a Nucleosil 100-7 column (functionalised with iminodiacetic acid (IDA) groups) was used to separate a mixture of transition metal complexes, manganese(II) and cobalt(II) in 4-(2-pyridylazo)-resorcinol (PAR). All optical measurements were taken by using both the in-built HPLC variable wavelength detector and the proposed paired-emitter-detector-diode (PEDD) optical detector configured in-line for data comparison. The concentration range investigated using the PEDD was found to give a linear response to the Mn(II) and Co(II) PAR complexes. The effects of flow rate and emitter LED light source intensity were investigated. Under optimised conditions the PEDD detector offered a linear range of 0.9–100 µM and LOD of 0.09 µM for Mn–PAR complex. A linear range of 0.2–100 µM and LOD of 0.09 µM for Co–PAR complex was achieved.

A sensor with potential for the development of a “chemical barcode” for real-time monitoring of fish freshness is described. This on-package sensor contains a pH sensitive dye, bromocresol green, that responds through visible colour change to basic volatile spoilage compounds, such as trimethylamine (TMA), ammonia (NH3) and dimethylamine (DMA) collectively known as Total Volatile Basic Nitrogen (TVB-N). The sensor characteristics were studied as well as its response with standard ammonia gas. Trials on cod and under-utilised species have verified that the sensor response correlates with bacterial growth patterns in fish samples thus enabling the “real-time” monitoring of spoilage in various fish species. The sensor response can be interrogated with a simple, inexpensive reflectance colorimeter that we have developed based on two light emitting diodes (LEDs) and a photodetector.

A disco photometer which was an optical sensing array based on multiple LEDs was constructed for colorimetric analysis. This approach has been used to analyse single dyes and dye mixtures containing up to three dye components. This technique made use of the inherent well-defined LED emission band to provide selectivity for chromophors, which have equally well-defined absorbance band to give very good analytical data. The results showed that this LED array configuration could be used to reduce the complexity of data obtained from the mixtures and also improve the quality of the output.

Calixarene L1 has been designed to behave as a fluorescent molecular sensor capable of distinguishing between chiral amines on the basis of their size and chirality. Fluorescence quenching studies of calixarene L1 in methanol demonstrated no enantiomeric selectivity for a short chain amino alcohol, phenylglycinol, while excellent selectivity was observed for a longer chain amino alcohol, phenylalaninol (PA). The effect of solvent on the fluorescent properties of this calixarene in the presence of PA has been studied, and demonstrates that varying solvent polarity allows the wavelength of enantiomer selectivity to be tuned from 227 nm to 440 nm. While enantiomeric selectivity is observed in methanol at 227 nm, no discrimination is achieved in acetonitrile. Chiral discrimination is statistically possible with L1 and PA in chloroform at 227 nm, but it is not comparable with the extent of discrimination achieved in methanol. In chloroform a new emission band at a longer wavelength (440 nm) is formed with R-PA in solution with L1, an effect that is not observed with the S-enantiomer. This new band in chloroform at 440 nm allows very effective chiral discrimination and has been attributed to the presence of two different conformations of calixarene L1, which is reinforced by 1H-NMR studies and molecular modelling studies.

A novel inexpensive optical-sensing technique has been developed for colorimetric flow analysis. This sensing system employs two LEDs whereby one is used as the light source and the other as a light detector. The LED used as light detector is reverse biased with a 5-V supply so that the photocurrent generated by the incident light discharges the capacitance. Direct digital output is provided by a simple timer circuit that measures the time taken for this discharge process from 5 V (logic 1) to 1.7 V (logic 0).This sensing concept has been applied in flow analysis by constructing an optical flow cell with a pair of LEDs. Calibration of the integrated optical flow cell using a dye resulted in a linear response that obeys the Beer–Lambert law. The flow rate, dynamic range, sensitivity and limits of detection were investigated. The system was also used for pH determination in the range of pH 2.5–6.8 using bromocresol green (BCG). The pKa of BCG was successfully determined by this technique.

This paper describes the implementation of a wireless sensor network for the temperature monitoring of shellfish catches over the Internet. Temperature loggers in shellfish boxes transmit data through radio frequency to a base station. The information is transferred to a server via the GSM network, where it is processed and uploaded to a database. The web-based interface allows configuration of the network and access to real-time and archived temperature data through any Internet-capable device. The system is designed to be completely autonomous, eliminating the need for repeated manual checks. Practical issues arising from implementation of the system are identified and performance of the system during field trials is assessed in comparison with commercially available temperature loggers.

The development of a novel, low power optical sensing platform based on light emitting diodes (LEDs) is described. The sensor is constructed from a pair of LEDs fused together at an angle where one LED functions as the light source and the other LED is reverse biased to function as a light detector. Sensor function is based on the level of light received by the detector diode, which varies with the reflectance of the interface between the device and its environment, or the chemochromic membrane that covers the device. A simple microprocessor circuit is used to measure the time taken for the photon-induced current to discharge the detector LED from an initial 5 V (logic 1) to 1.7 V (logic zero). This sensing device has been successfully used for colour and colour-based pH measurements and offers extremely high sensitivity, enabling detection down to the sub micro molar level of dyes.

Progress in the development of a miniaturised microfluidic instrument for monitoring phosphorus in natural waters and wastewater is presented. The yellow colorimetric method for phosphate analysis has been transferred to a microfluidic chip configuration This simple method employs one reagent mixed in a 1∶1 ratio with a sample to produce a yellow colour absorbing strongly below 400 nm. A stopped flow approach is used which, together with the very rapid kinetics and simple reagent stream, enables a very uncomplicated microfluidic manifold design to be adopted. The working wavelength is 380 nm to coincide with the peak output of a recently developed UV-LED narrow bandwidth light source. The limit of detection for the yellow method is 0.2 ppm with a dynamic linear range from 0–50 ppm possible. The reaction time at room temperature is less than 3 min, which means that up to 20 samples per hour can be analysed.

The release of amines from decomposing fish such as trimethylamine (TMA), dimethylamine (DMA) and ammonia, collectively known as total volatile basic nitrogen (TVB-N), are in high enough concentrations in headspace to be monitored by a colour change in a pH-sensitive sensor. A method developed here uses a pH indicator dye physically trapped in a cellulose polymer film to respond to the headspace TVB-N released from selected fish species during spoilage. Two species were selected for analysis on the basis of economic importance and the levels of volatile amines released were followed with time using uv/vis reflectance spectroscopic measurements. The results show that there is a significant increase in the TVB-N content in the headspace of fish samples after an incubation period of 8–12 h for cod and 12–15 h for orange roughy.

The concept of micro total analysis systems (μTAS) or Lab-on-a-chip is based on the twin strategies of integration and miniaturisation that have been so successful in the electronics industry. This paper will look at the materials issues, particularly with respect to the new polymeric materials that are becoming available, and strategies for integrating optical (colorimetric) detection. The influence of breakthroughs in apparently unrelated areas on the range of chemistries that can be applied will be illustrated. For environmental monitoring, the further integration of wireless communications with micro-dimensioned analytical instruments and sensors will become the ultimate driving force. The emergence of these compact, self-sustaining, networked instruments will have enormous impact on all field-based environmental measurements.

The application of a phosphorus monitoring device based on microsystems technology (MST) to the analysis of river water is presented. An alternative to the standard molybdenum blue method known as the yellow vanadomolybdophosphoric acid method has been very effectively implemented. The method is simple, a reagent and sample are mixed in a 1∶1 ratio forming a yellow complex that absorbs strongly below 400 nm in the UV spectrum. The kinetics of the reaction are rapid and sample turnaround is typically 3 min at room temperature. Therefore a very uncomplicated microfluidic design can be adopted. The working wavelength was chosen as 380 nm to coincide with the peak output of a UV-LED narrow bandwidth light source recently developed by Nichia. The limit of detection for the yellow method in the microfluidic system is 0.2 ppm with a dynamic linear range from 0–50 ppm. The method was applied to a measurement of phosphorus in a local river at specific sampling points along its course.

This paper presents new analytical data, and evidence of the fluorescence quenching mechanism involved in enantioselective signals obtained with the chiral host tetra-(S)-di-2-naphthylprolinol calix[4]arene. Excellent selectivity is obtained with phenylglycinol and norephidrine in methanol, and samples of unknown enantiomeric composition have been determined with an average error of 1.08 and 0.62%, respectively (n = 6), on the basis of a single fluorescence measurement. The absence of any quenching observed with the related amines of phenylalaninol is attributed to a methylene spacer which inhibits efficient interaction between the aryl group of the guest and the naphthyl group of the host. The importance of the phenyl group of the guest in the quenching mechanism is confirmed by the complete absence of any quenching effect with cyclohexylethylamine.

The release of volatile amines such as trimethylamine (TMA), dimethylamine (DMA) and ammonia from fish samples has been detected and continuously monitored through changes in the colour of a sensitive acidochromic dye using UV–Vis reflectance spectroscopy. Changes in the colour of the calix[4]arene-based dye immobilised on test paper disks in contact with the headspace of cod and whiting samples could be determined by an increase in absorbance centred on ca. 500–510 nm. Fresh whiting produces larger, and more rapid colour changes than fresh cod, suggesting that the rate of release of volatile bases from whiting is faster. The sensitivity of the dye response can be tuned by varying the ratio of LiClO4 to dye in the dye solution due to the formation of a more acidic Li+–dye complex. Volatile components absorbing at lower wavelengths (below 410 nm) were detected in cod samples but not whiting.

A new series of LCST ILs have been copolymerised with crosslinkers of varying length to afford the first ever thermoresponsive poly(ionic liquid)-based hydrogels. These hydrogels exhibit surprisingly broad LCST and volume transition behaviour compared to standard thermoresponsive gels and linear ILs.

The ability to switch the physico-chemical properties of conducting polymers opens up new possibilities for a range of applications. Appropriately functionalised materials can provide routes to multi-modal switching, for example, in response to light and/or electrochemical stimuli. This capability is important in the field of bionics wherein remote and temporal control of the properties of materials is becoming attractive. The ability to actuate a film via photonic stimuli is particularly interesting as it facilitates the modulation of interactions between host binding sites and potential guest molecules. In this work, we studied two different poly-terthiophenes: one was functionalised with a spiropyran photoswitch (pTTh-SP) and the second with a non-photoswitchable methyl acetate moiety (pTTh-MA). These substrates were exposed to several cycles of illumination with light of different wavelengths and the resulting effect studied with UV-vis spectroscopy, contact angle and atomic force microscopy (AFM). The AFM tips were chemically activated with fibronectin (FN) and the adhesion force of the protein to the polymeric surface was measured. The pTTh-MA (no SP incorporated) showed a slightly higher average maximum adhesion (0.96 ± 0.14 nN) than the modified pTTh-SP surface (0.77 ± 0.08 nN), but after exposure of the pTTh-SP polymer to UV, the average maximum adhesion of the pTTh-MC (merocyanine form) was significantly smaller (0.49 ± 0.06 nN) than both the pTTh-MA and pTTh-SP. In addition, the tip-sample separation distances of the adhesive interactions are indicative of the FN interaction occurring over a distance more closely related to the average dimensions of its compact conformation. The results suggest that surface energy and hydrophobic forces are predominant in determining the protein adhesion to the films studied and that this effect can be photonically tuned. By extension, this further implies that it should be possible to obtain a degree of spatial and temporal control of the surface binding behaviour of certain proteins with these functionalised surfaces through photo-activation/deactivation, which, in principle, should facilitate patterned growth behaviour (e.g. using masks or directional illumination) or photocontrol of protein uptake and release.

The physicochemical properties of free-standing cross-linked poly(N-isopropylacrylamide) (pNIPAAM) gels, generated in the presence of the Ionic liquids (ILs), 1-ethyl-3-methylimidazolium [C2mIm]+ salts of ethylsulfate [EtSO4]−, dicyanamide [DCA]−, bis(trifluoromethylsulfonyl)imide [NTf2]−, and trihexyltetradecylphosphonium dicyanamide ([P6,6,6,14][DCA]) are described. The Lower Critical Solution Temperature (LCST) of the resulting ionogel was found to vary between 24–31 °C. The behaviour of swelling is found to be as high as 31.55% (±0.47, n = 3) from the initial dehydrated state, while 28.04% (±0.42, n = 3) shrinking from the hydrated swollen state is observed. For ionogels based on the [DCA]− anion an unexpected complete loss of the shrinking behaviour occurs, attributed to water interactions with the nitrile group of the [DCA]− anion. Scanning Electron Microscopy also reveals distinct morphological changes, for example [C2mIm][EtSO4] displays a highly porous, nodule type morphology, efficiently pre-disposed for water uptake.

The photochemistry and thermodynamics of two terthiophene (TTh) derivatives bearing benzospiropyran (BSP) moieties, 1-(3,3″-dimethylindoline-6′-nitrobenzospiropyranyl)-2-ethyl 4,4″-didecyloxy-2,2′:5′,2″-terthiophene-3′-acetate (BSP-2) and 1-(3,3″-dimethylindoline-6′-nitrobenzospiropyranyl)-2-ethyl 4,4″-didecyloxy-2,2′:5′,2″-terthiophene-3′-carboxylate (BSP-3), differing only by a single methylene spacer unit, have been studied. The kinetics of photogeneration of the equivalent merocyanine (MC) isomers (MC-2 and MC-3, respectively), the isomerisation properties of MC-2 and MC-3, and the thermodynamic parameters have been studied in acetonitrile, and compared to the parent, non-TTh-functionalised, benzospiropyran derivative, BSP-1. Despite the close structural similarity of BSP-2 and BSP-3, their physicochemical properties were found to differ significantly; examples include activation energies (Ea(MC-2) = 75.05 kJ mol−1, Ea(MC-3) = 100.39 kJ mol−1) and entropies of activation (ΔS‡MC-2 = 43.38 J K−1 mol−1, ΔS‡MC-3 = 37.78 J K−1 mol−1) for the thermal relaxation from MC to BSP, with the MC-3 value much closer to the unmodified MC-1 value (46.48 J K−1 mol−1) for this latter quantity. The thermal relaxation kinetics and solvatochromic behaviour of the derivatives in a range of solvents of differing polarity (ethanol, dichloromethane, acetone, toluene and diethyl ether) are also presented. Differences in the estimated values of these thermodynamic and kinetic parameters are discussed with reference to the molecular structure of the derivatives.

The photo-, thermo- and solvatochromic properties of 2,3-dihydro-1′,3′,3′-trimethyl-6-nitrospiro-[1-benzopyran-2,2′-1H-indole] (BSP-NO2) were studied in ILs containing the anion [NTf2]− by UV-Vis absorption spectroscopy, ab initio molecular orbital theory and density functional theory (DFT) calculations. It was found that the kinetics and thermodynamics of the BSP-NO2 ↔ MC (merocyanine) equilibrium was sensitive to the nature of the cation. It was also observed that the imidazolium cation can form a through-space orbital interaction with the MC isomer, rather than a simple electrostatic interaction, thus preventing the MC conversion back to the BSP-NO2 isomer. The BSP-NO2 ↔ MC equilibrium thus serves as a model system for studying modes of interaction of the cations in ionic liquids.

We have investigated the kinetic and thermodynamic parameters of thermal reversion of the spirooxazine 1,3,3-trimethyl-5′-(2-benzothiazolyl)-spiroindoline-2,3′-naphtho(2,1-b)(1,4) oxazine (SO) in molecular solvents and ionic liquids containing the [NTf2]− anion. ET(30) and Kamlet–Taft parameter studies were employed to interpret IL polarity and attempt to rationalise the kinetic–polarity relationship of MC→SO relaxation. The observed thermal relaxation of SO within ionic liquids was slower than that of molecular solvents with similar polarity, indicating a greater degree of interactions between the ionic liquid ions and the zwitterionic MC isomer, which led to increased lifetimes for the MC–ion complexes (19.6 s in acetonitrile and 90.9 s in [P6,6,6,14][NTf2]). Thermodynamic parameters of activation implied enhanced ordering of SO in ILs and its subsequent effect on thermal reversion of SO. Activation parameters found MC→SO relaxation was more temperature dependent in polar protic ILs such as [bmIm]+ than aprotic ILs such as [P6,6,6,14][NTf2] with ΔS‡ values of 40 J K mol−1 and −8.6 J K mol−1, respectively. Activation energies were generally higher in ILs (83.9–97 kJ mol−1) compared to molecular solvents (67.8–89.8 kJ mol−1), which was reflected in longer lifetimes of MC due to greater energy barriers of relaxation to SO. Pre-metathesis cleaning of precursor salts was found to be necessary in order to obtain spectroscopic grade ILs for physicochemical analysis using solvatochromic probe dyes. Inconsistencies in the polarity–kinetic relationship further reinforced the belief that measuring polarity of ILs may not be possible with solvatochromic probe dyes.

We investigate the physicochemical properties of a novel imidazolium benzospiropyran derivative, SPIm, in imidazolium based ionic liquids (ILs). SPIm was prepared through alkylation of an imidazole to the photoswitchable compound and this derivative was characterised in imidazolium based ILs with increasing chain length to examine the stability of its merocyanine (MC) and spiropyran (SP) forms and compared to standard spiropyran, BSP. The rate of thermal relaxation of the new derivative is found to be about ten times faster than that of BSP as reflected in rates of 13.9 × 10−3 s−1 and 1.0 × 10−3 s−1 for SPIm and BSP, respectively, in [C6mIm][NTf2]. Since ILs are believed to form nano-structured domains it is proposed that the covalent attachment of the imidazolium side group of SPIm fully integrates the photoswitchable moiety into the non-polar region through side-chain association. In contrast, unbound BSP is relatively free to migrate between both polar and non-polar regions and the MC form is more readily stabilised by the IL charge via through space interactions and spontaneous movement to charged nano-domains leading to enhancement of the MC lifetime. At higher concentrations, rheological and transport properties were investigated to determine the impact of covalent attachment of the BSP fragment to an imidazolium cation on the ionic liquid structure. Ionic conductivity was found to decrease by up to 23% for SPIm with effects increasing with cation side-chain length. Unlike BSP, the photoswitching of the SPIm did not affect conductivity or viscosity values. This may indicate that the mobility of the photoswitchable compound and the resulting disruption of such movement may be critical to the control of this physical property.

Spiropyran derivatives have been immobilised on the surface of polystyrene microbeads using different immobilisation strategies. These functionalised polymeric beads can be reversibly switched between the colourless inactive spiropyran (SP) and highly coloured (purple) active merocyanine (MC) forms using low power light sources, such as light-emitting diodes (LEDs). A UV LED (375 nm) is used for the SP → MC conversion, and a white LED (430–760 nm) for the reverse MC → SP conversion. The photochromic behaviour of the coated beads has been characterised using different LEDs and reflection spectroscopy, employing optic fibres and an in-house-designed holder. Investigations into the metal-ion binding behaviour of the spiropyran-modified microbeads have shown that Cu2+ ions cause an appreciable colour and spectral change when brought into contact with the beads in the MC form, suggesting that a significant interaction is occurring. However, the Cu2+ ions can be completely expelled by photonic-conversion of the beads into the inactive SP form using a white LED. This sequence has been successfully repeated six times, suggesting that it is possible to cycle through activation of the functionalised beads from a non-binding to a binding form (SP → MC) using a UV LED, allow binding with Cu2+ ions to occur, and subsequently, expel the bound ions and regenerate the passive SP surface using a white LED. Other metals, such as calcium, do not cause any appreciable colour or spectral change over the same concentration range and in the presence of the same anion (final concentration 7.1 × 10−4 M nitrate salt in ethanol). The system is therefore self-indicating in terms of whether the active MC or inactive SP forms are present, and whether Cu2+ ions are bound to the MC form. In principle, therefore, these functionalised beads could form the basis of a photoswitchable stationary phase for metal ion binding and detection: irradiation of the stationary phase with UV LEDs causes retention of guest species due to the presence of the MC form, while subsequent exposure to white LEDs causes release of guest species into the mobile phase.

Here we describe, for the first time, intriguing solvato-morphological control of spiropyran-based microcrystalline structures. These microstructures exhibit reversible photoisomerization upon light irradiation (UV/Vis) in the solid-state. Finally, light-guided aggregation of these microstructures at the liquid/air interface is also demonstrated.

A crosslinked poly(N-isopropylacrylamide) ionogel encapsulating an ionic liquid exhibits improved transmittance properties, enhanced water uptake/release, greater thermal actuation behaviour and distinct solvatomorphology over its hydrogel equivalent. It was also found that the rate of release of fluorescein pre-loaded into membranes was considerably enhanced for ionogels compared to equivalent hydrogels, and could be triggered through changes in pH and temperature.

This work describes the first use of a direct nitrate analyser using chromotropic acid. A simplified chromotropic acid method eliminating several steps previously associated with this method is employed in the platform. In a sulphuric acid medium, chromotropic acid reacts with nitrate ions and produces a characteristic yellow colour associated with an absorbance band in the visible region (λmax = 430 nm). The modified method allows for nitrate determination over the linear range 0.9–80 mg L−1 nitrate with a limit of detection of 0.73 μg L−1 nitrate. Validation was achieved by analysing water samples from various sources including groundwater, trade effluent and drinking water by the modified method and by ion chromatography. The method was implemented on a flow analysis platform incorporating a paired emitter–detector diode (PEDD) as the optical detector. An excellent correlation coefficient of 0.993 was obtained between the modified method and ion chromatography. The modified chromotropic acid method represents a rapid, simple, low cost technique for the direct determination of nitrate in water.

Progress towards the development of a miniaturised microfluidic instrument for the direct measurement of nitrate in natural waters and wastewater using chromotropic acid is presented. For the first time, the chromotropic method for nitrate analysis has been transferred to a microfluidic chip configuration that can withstand the extremely acidic nature of the reagent within a field deployable platform. This simple method employs one reagent mixed in a 1:1 ratio with the sample to produce a yellow colour absorbing strongly at 430 nm. A stopped flow approach is used which, together with the very rapid kinetics and simple reagent stream, enables an uncomplicated microfluidic design and field deployable platform with a sample throughput of 9 samples per h, limits of detection of 0.70 mg L−1 NO3− and 0.31 mg L−1 NO3− for seawater samples, with a dynamic linear range from 0–80 mg L−1 NO3− and long-term reagent stability of up to 6 months. Validation was achieved by analysing split water samples by the analyser and ion chromatography, resulting in an excellent correlation co-efficient of 0.9969. The fully integrated sensing platform consists of a sample inlet with filter, storage units for chromotropic reagent and standards for self-calibration, pumping system which controls the transport and mixing of the sample, a microfluidic mixing and detection chip, and waste storage, all contained within a ruggedized, waterproof housing. The optical detection system consists of a LED light source with a photodiode detector, which enables sensitive detection of the coloured complex formed. The low cost of the platform coupled with integrated wireless communication makes it an ideal platform for in situ environmental monitoring.