•Cobalt(II)-doped carbon dots (CCDs) have been fabricated.•CCDs were used for Cr(VI) sensing based on photoluminescence quenching.•The structural characterization of as-obtained CCDs was thoroughly investigated.•The CCDs was capable of rapidly detecting Cr(VI) in tap water and fish samples.A new type of cobalt(II)-doped carbon dots (CCDs) have been fabricated and used successfully for sensing Cr(VI) ions on the basis of photoluminescence quenching. The structural characterization of as-obtained CCDs was thoroughly performed by transmission electron microscopy (TEM), Fourier transform infrared (FT-IR) and X-ray photoelectron (XPS) spectroscopy. The optical properties were also determined by absorption and fluorescence spectra. By recording 3D fluorescence spectrum, a unique intra-particle Förster resonance energy transfer (FRET) system was investigated. In addition, fluorescence quenching of CCDs was observed in the presence of Cr(VI) ions due to inner filter effect. A good linear relationship between the concentration of Cr(VI) ions and fluorescent intensity was obtained in the range from 5 μM to 125 μM (R2=0.99), and the limit of detection was calculated as 1.17 μM (0.12 ppm for Cr(VI)). Importantly, this method was capable of rapidly detecting Cr(VI) ions in tap water and fish samples, which may be helpful in risk reduction of intake Cr(VI) contamination from water and seafood.

•The BODIPY monomers self-assemble to form nonfluorescent NPs.•BODIPY-based self-assembled nanoparticles were used as fluorescence turn-on sensor.•Zn2+ in human hair was quantitatively detected.•The detection limit for Zn2+ determination was calculated as low as 61.3 nM.Zinc plays important roles in regulating physiological and pathological processes. Regrettably, mild to moderate zinc deficiency is common worldwide. Hair Zn2+ concentration, which reflects a zinc storage status, is useful for tracking trends in zinc status within populations. In this work, we report BODIPY-based self-assembled nanoparticles (NPs) as fluorescence turn-on sensor for the selective sensing of Zn2+ in human hair. The BODIPY monomers (BAN) self-assemble in aqueous medium to form nonfluorescent NPs. In the presence of Zn2+ ions, the NPs selectively show an obvious turn-on fluorescence change. This selective response of the NPs allows the determination and quantification of Zn2+ in human hair with a detection limit of 61.3 nM. This study demonstrates that the small molecule self-assembled nanoparticle is a versatile and useful tool, and shows great potential for applications in sensing of important analytes in biological systems.

In this work, Cu2O nanorods modified by reduced graphene oxide (rGO) were produced via a two-step synthesis method. CuO rods were firstly prepared in graphene oxide (GO) solution using cetyltrimethyl ammonium bromide (CTAB) as a soft template by the microwave-assisted hydrothermal method, accompanied with the reduction of GO. The complexes were subsequently annealed and Cu2O nanorods/rGO composites were obtained. The as-prepared composites were evaluated using various characterization methods, and were utilized as sensing materials. The room-temperature NH3 sensing properties of a sensor based on the Cu2O nanorods/rGO composites were systematically investigated. The sensor exhibited an excellent sensitivity and linear response toward NH3 at room temperature. Furthermore, the sensor could be easily recovered to its initial state in a short time after exposure to fresh air. The sensor also showed excellent repeatability and selectivity to NH3. The remarkably enhanced NH3-sensing performances could be attributed to the improved conductivity, catalytic activity for the oxygen reduction reaction and increased gas adsorption in the unique hybrid composites. Such composites showed great potential for manufacturing a new generation of low-power and portable ammonia sensors.

Fluorescence quenching induced by targets is always an alluring strategy to elucidate the possible photoluminescence origin of carbon dots. In this study, a new kind of N, S co-doped carbon dots (NSCDs) was synthesized and the fluorescence of NSCDs was surprisingly found to be quenched by chlorophenols (CPs) in a pKa dependent mode. Detailed investigation of this behavior demonstrated that phenolate was the responsible species and N and/or S dopants in NSCDs failed to play a role in the fluorescence quenching. Further evidence uncovered that the quenching was a static one, where a non-fluorescent intermediate was formed between electron-deficient CO on the CDs surface and the electron-rich phenolic oxygen anion of chlorophenolate via nucleophilic addition. Moreover, one of the main photoluminescence origins of this kind of CDs was derived, namely surface emissive sites mostly attributed to carbonyl groups.

Cysteine (Cys), homocysteine (Hcy), and glutathione (GSH) are interconnected and play essential roles for regulating the redox balance of biological processes. However, finding a simple and effective method for the simultaneous determination for these three biothiols in biological systems is always a challenge. In this work, we report a method for the simultaneous quantitative determination of three biothiols in a mixture using a monochlorinated boron dipyrromethene (BODIPY)-based fluorometric sensor. At a specified period of time, after reacting with excess sensor, Hcy and GSH form predominantly sulfur-substituted BODIPY, while Cys generates sulfur-amino-diBODIPY due to a fast substitution–rearrangement–substitution reaction. A significant difference in polarities of these respective major products simplifies their separation by TLC, thus leading to the simultaneous determination of Cys, Hcy, and GSH readily. The sensor was successfully applied for the simultaneous quantitative detection of three biothiols in human serum, and the results were in good agreement with those obtained via high performance liquid chromatography (HPLC).Keywords: BODIPY; cysteine; glutathione; homocysteine; serum

The precisely controlled synthesis of a specific morphology is proved to be highly effective for enhancing sensing performance of functional micro- and nano-materials. Thus, a comprehensive understanding of the effect of synthesis conditions on morphology evolution is of great importance. In this paper, by using In2O3 as an example, we demonstrate the great effect of supersaturation on the morphology evolution during the growth process. Under vapour-phase synthesis, by varying positions of product collection, the amount of the starting materials, and reaction temperatures, a continuous two-step route consisting of ammonolysis and re-oxidation processes was developed for the morphology evolution of several kinds of In2O3, including octahedron strings, nanowires, crystal chains, lollipop-like structures, and particles. The controlled supersaturation of the In2O3 vapour was eventually proved to be a decisive factor for the formation of various In2O3 morphologies. Furthermore, we evaluated the gas-sensing properties of the three kinds of one-pot synthesized In2O3, indicating a shape-dependent relationship with sensing performance. The present work not only offers a unique strategy to control supersaturation in vapour-phase synthesis, but also provides an opportunity to deeply understand the morphology-dependent sensing performance.

•A novel electronic nose was developed using only one type of semiconductor metal oxide.•The electronic nose was based on the resistometric semiconductor metal oxide sensor array.•Porous In2O3 microtubes offered great surface area and large gas penetration channels.•Four alcohols at same concentrations were distinguished using electronic nose.•14 VOCs at their IDLH and PEL concentrations were discriminated using electronic nose.We have innovatively developed an electronic nose consisting of only one type of semiconductor metal oxide (SMO) material. The representative SMO material, porous In2O3 microtubes in this work, offered great surface area and large gas penetration channels. By using a solvent casting process, different amounts of porous In2O3 microtubes were coated on Al2O3 substrate, forming a resistometric SMO sensor array-based electronic nose. Each sensing unit in the electronic nose exhibited independent response toward ethanol. We have successfully applied this electronic nose to distinguish four alcohols at the same concentrations (100 ppm), and also utilized the electronic nose for the discrimination of 14 volatile organic compounds (VOCs). Clear differentiation among all the 14 VOCs both at their immediately dangerous to life or health (IDLH) and the permissible exposure limit (PEL) concentrations has been achieved with no errors or misclassifications. We expect that this method will expand the application of SMO sensor array-based electronic nose which has been largely limited by the selection of commercially available SMOs and dopants.

Optical sensor arrays typically require a large set of chemically responsive colorants to enhance discrimination capability. Conversely, we have proven that by using multiple concentrations of one indicator, the discrimination of various analytes could be realized.

In this report, we developed an additive-free synthesis of In2O3 cubes embedded into graphene networks with InN nanowires (InN-NWs) and graphene oxide (GO) as precursors by a facile one-step microwave-assisted hydrothermal method. In absence of GO, the InN-NWs maintained their chemical composition and original morphology upon the same treatment. At varying mass ratios of InN-NWs and GO, the different morphologies and distributions of In2O3 could be obtained on graphene sheets. The uniform distribution, which is usually considered favorable for enhanced sensing performance, was observed in In2O3 cubes/reduced graphene oxide (rGO) composites. The room-temperature NO2 sensing properties of the In2O3 cubes/rGO composites-based sensor were systematically investigated. The results revealed that the sensor exhibited a significant response to NO2 gas with a concentration lower to 1 ppm, and an excellent selectivity, even though the concentrations of interferential gases were 1000 times that of NO2. The enhanced NO2 sensing performances were attributed to the synergistic effect of uniformly distributed In2O3 cubes and graphene sheets in the unique hybrid architectures without the interfering of extra additives.Keywords: graphene oxide; In2O3 cubes; precursor; sensing performance; synergistic effect

The sensitive determination of heavy-metal ions has been widely investigated in recent years due to their threat to the environment and to human health. Among various analytical detection techniques, inexpensive colorimetric testing papers/strips play a very important role. The limitation, however, is also clear: the sensitivity is usually low and the selectivity is poor. In this work, we have developed a postage stamp-sized array sensor composed of nine commercially available heterocyclic azo indicators. Combining filtration-based enrichment with an array of technologies-based pattern-recognition, we have obtained the discrimination capability for seven heavy-metal ions (Hg2+, Pb2+, Ag+, Ni2+, Cu2+, Zn2+, and Co2+) at their Chinese wastewater discharge standard concentrations. The allowable detection level of Hg2+ was down to 0.05 mg L−1. The heavy-metal ions screening test was readily achieved using a standard chemometric approach. And the array sensor applied well in real water samples.

By using sensing technology, the individual component analysis at trace level in complex samples remains problematic simply because of various interfering species. For example, the determination of Cd2+ in rice is difficult due to the co-existing interfering metal cations at thousands or even millions of times higher concentrations. In this study, a heavy-metal ion sensitive BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-based fluorometric paper sensor with assistance of solid phase extraction (SPE) was developed for the highly selective and sensitive determination of trace Cd2+ in rice. SPE column packed with prepared sulfonated PS–DVB microspheres was employed to enrich trace Cd2+ and meanwhile remove most interfering heavy-metal ions in simulated complex rice sample with oxalic acid as eluent, which was theoretically selected on the basis of f values. Mn2+, as a major coexistent heavy-metal ion, could not be easily removed by SPE, but showed little fluorescent response for BODIPY fluorometric paper sensor even in excess amounts. Combining the separation and enrichment capability of SPE column with the selectivity of BODIPY-based fluorometric paper sensor, we were able to detect trace Cd2+ in complex samples. The response of fluorometric paper sensor was linearly related with Cd2+ concentrations in the range of 0.5–4 μM, with a detection limit of 0.5 μM. Twelve real rice samples spiked with Cd2+ were analysed using this method and the results are in good agreement with ICP-MS measurements.

•Indicators based on BODIPY and di-2-picolyamine derivatives were designed.•12 cross-reactive BODIPY indicators provide facile identification of the heavy metal ions.•The collected images were digitized for the semi-quantitative discriminations.•Array technologies and pattern-recognition were combined.A BODIPY(4,4-difluoro-4-bora-3a,4a-diaza-s-indacene)-based fluorometric sensor array has been developed for the highly sensitive detection of eight heavy-metal ions at micromolar concentration. The di-2-picolyamine (DPA) derivatives combine high affinities for a variety of heavy-metal ions with the capacity to perturb the fluorescence properties of BODIPY, making them perfectly suitable for the design of fluorometric sensor arrays for heavy-metal ions. 12 cross-reactive BODIPY fluorescent indicators provide facile identification of the heavy-metal ions using a standard chemometric approach (hierarchical clustering analysis); no misclassifications were found over 45 trials. Clear differentiation among heavy-metal ions as a function of concentration was also achieved, even down to 10−7 M. A semi-quantitative interpolation of the heavy-metal concentration is obtained by comparing the total Euclidean distance of the measurement with a set of known concentrations in the library.A BODIPY-based fluorometric sensor array has been developed for the highly sensitive detection of eight heavy metal ions at micromolar concentration. 12 cross-reactive BODIPY fluorescent indicators provide facile identification of the heavy metal ions using a standard chemometric approach (hierarchical clustering analysis); no misclassifications were found over 45 trials. Clear differentiation among heavy metal ions as a function of concentration was also achieved, even down to 10−7 M.

The optical colorimetric/fluorometric sensor array has been applied widely in many fields for its low-cost, easy-operation, fast-response, multi-information, etc. This review describes the progresses in recent years in the optical colorimetric/fluorometric sensor array and summarizes its applications in the detection of toxic gases, biological samples, ions, small molecules in solution, and complex mixtures. Moreover, the characteristics and performances of different colorimetric/fluorometric sensor arrays are discussed and the potential directions for their future research are prospected as well.

A fluorometric paper-based sensor array has been developed for the sensitive and convenient determination of seven heavy-metal ions at their wastewater discharge standard concentrations. Combining with nine cross-reactive BODIPY fluorescent indicators and array technologies-based pattern-recognition, we have obtained the discrimination capability of seven different heavy-metal ions at their wastewater discharge standard concentrations. After the immobilization of indicators and the enrichment of analytes, identification of the heavy-metal ions was readily acquired using a standard chemometric approach. Clear differentiation among heavy-metal ions as a function of concentration was also achieved, even down to 10−7 M. A semi-quantitative estimation of the heavy-metal ion concentration was obtained by comparing color changes with a set of known concentrations. The sensor array was tentatively investigated in spiked tap water and sea water, and showed possible feasibility for real sample testing.Highlights► Fluorometric indicators were synthesized combining the BODIPY and multipyridyl receptors. ► Chemometric analysis was successfully applied. ► Heavy-metal ions were discriminated at their wastewater discharge standard concentrations. ► Indicators were immobilized and a simple paper-based sensor was designed.

The analysis of anions in water presents a difficult challenge due to their low charge-to-radius ratio, and the ability to discriminate among similar anions often remains problematic. The use of a 3 × 6 ratiometric indicator-displacement assay (RIDA) array for the colorimetric detection and identification of ten anions in water is reported. The sensor array consists of different combinations of colorimetric indicators and metal cations. The colorimetric indicators chelate with metal cations, forming the color changes. Upon the addition of anions, anions compete with the indicator ligands according to solubility product constants (Ksp). The indicator–metal chelate compound changes color back dramatically when the competition of anions wins. The color changes of the RIDA array were used as a digital representation of the array response and analyzed with standard statistical methods, including principal component analysis and hierarchical clustering analysis. No confusion or errors in classification by hierarchical clustering analysis were observed in 44 trials. The limit of detection was calculated approximately, and most limits of detections of anions are well below μM level using our RIDA array. The pH effect, temperature influence, interfering anions were also investigated, and the RIDA array shows the feasibility of real sample testing.Graphical abstractA colorimetric indicator-displacement assay (IDA) array has been developed for the determination of ten anions in water. The color changes in IDA array provide facile identification of these anions with no misclassification.Highlights► The RIDA array was developed to sense ten anions in aqueous solution. ► No complicated molecular synthesis is needed. ► The collected images were digitized for the semi-quantitative discriminations. ► Array technologies and pattern-recognition were combined. ► The transparency scan unit was used to avoid the light reflection.

A novel rhodamine–quinoline derivative-based indicator for Cu2+ ion determination was designed and synthesized. It exhibited highly selective and sensitive colorimetric and “turn-on” fluorescent responses toward Cu2+ ions based on the ring-opening mechanism of the rhodamine spirolactam in aqueous solution. The colorimetric and fluorescent responses were recorded using a domestic scanner and camera-based home-made fluorescent imaging unit, separately. The images were digitized, and the red (R), green (G), and blue (B) values were investigated. Both colorimetric and fluorescent methods showed good selectivity, and the color/fluorescence changes were remarkable for the Cu2+ ion detection even in the presence of other metal ions. The good linear relationship was easily obtained between the color/fluorescence changes and the concentrations in the range of 20–120 μM.

A colorimetric filtration method has been developed for the highly selective and sensitive determination of Ni2+ and Pb2+ ions. Determinations of Ni2+ and Pb2+ follow the filtration using nioxime (1,2-cyclohexanedione dioxime) and rhodizonic acid disodium salt, respectively, as colorimetric reagents. Different from regular instrumentation techniques, the metal chelate precipitations are continuously pumped into a home-made flow cell at a constant flow rate, and filtered by a cellulose acetate/nitrate membrane. The color changes of the membrane are imaged using a conventional flatbed scanner, and digitized. The special selection of individual channels in the red, green, and blue channels of the images filters the influences of coexisting ions and provides a highly selective detection of Ni2+ and Pb2+ cations. The linear relationship between the colorimetric response of the chosen channel and Ni2+ or Pb2+ concentrations indicates a quantitative detection. The detection limit for Pb2+ is 3 μM (almost half of the Chinese wastewater discharge standard concentration), and is well below the nM level (94 nM) for Ni2+ (a quarter of the WHO drinking water safe-exposure standard for Ni2+). The determinations take five to ten minutes. No shelf life issue exists because the chelating indicators react with metal directly without any pre-immobilizations.

A ratiometric indicator-displacement assay (RIDA) array has been developed for the semi-quantitative colorimetric determination of chloride, bromide, and iodide anions. Determinations of these halide anions follow the displacement reaction using the chelate compound of (2-(3,5-dibromo-2-pyridylazo)-5-(diethylamino)phenol) (3,5-Br2-PADAP) and heavy metal salts as colorimetric reagent. Different from regular silver nitrate titrations, the chloride, bromide, and iodide anions compete with the 3,5-Br2-PADAP ligand and change the colour of the 3,5-Br2-PADAP–metal chelate compound dramatically. These clearer colour changes make the semi-quantitative colorimetric determination of chloride, bromide, and iodide anions possible. The colour changes are imaged using a conventional flatbed scanner, and digitized. After statistical analysis, these colour changes in the RIDA array provide facile identification of chloride, bromide, and iodide anions at a wide concentration range (10 μM to 10 mM) without any misclassification. The RIDA array is able to discriminate without misclassifications among seven concentrations of chloride, bromide, and iodide anions. No shelf life issue exists because the chelating compounds react with halide anions directly without any pre-immobilizations.

A simple and sensitive colorimetric method for the determination of trace copper(II) ions in aqueous solution has been developed using diphenylcarbazide (DPC) immobilized sol–gel matrices. To enhance the odds of chelating interactions between copper(II) ions and DPC, a peristaltic pump was utilized to drive copper(II) ions solution pass through the cellulose acetate/nitrate membrane, which was coated with DPC immobilized sol–gel matrices. The membrane was sandwiched in a home-made flow cell. The porous silica matrix serves as a preconcentrator, and enriches the trace copper(II) ions. Meanwhile, the filtration increases the chelating interaction probability between copper(II) ions and DPC, thereby improves the sensitivity significantly. After the formation of purple complex compound, the color changes of the DPC immobilized sol–gel membrane were recorded using a flatbed scanner. The images were digitized, and the red (R), green (G), and blue (B) values were investigated. The colorimetric method provides a rapid and reliable determination of trace copper(II) ions with a detection limit as low as 0.16 μM and a kinetic range up to 1.6 μM in neutral medium. Moreover, the method shows good selectivity, and the color changes of the DPC immobilized sol–gel membranes are remarkable for the copper(II) ions detection even in the presence of other metal ions.

Optical sensor arrays typically require a large set of chemically responsive colorants to enhance discrimination capability. Conversely, we have proven that by using multiple concentrations of one indicator, the discrimination of various analytes could be realized.

In this work, Cu2O nanorods modified by reduced graphene oxide (rGO) were produced via a two-step synthesis method. CuO rods were firstly prepared in graphene oxide (GO) solution using cetyltrimethyl ammonium bromide (CTAB) as a soft template by the microwave-assisted hydrothermal method, accompanied with the reduction of GO. The complexes were subsequently annealed and Cu2O nanorods/rGO composites were obtained. The as-prepared composites were evaluated using various characterization methods, and were utilized as sensing materials. The room-temperature NH3 sensing properties of a sensor based on the Cu2O nanorods/rGO composites were systematically investigated. The sensor exhibited an excellent sensitivity and linear response toward NH3 at room temperature. Furthermore, the sensor could be easily recovered to its initial state in a short time after exposure to fresh air. The sensor also showed excellent repeatability and selectivity to NH3. The remarkably enhanced NH3-sensing performances could be attributed to the improved conductivity, catalytic activity for the oxygen reduction reaction and increased gas adsorption in the unique hybrid composites. Such composites showed great potential for manufacturing a new generation of low-power and portable ammonia sensors.