•We developed a colorimetric detection of glucose based on AuNPs coupled with AgNPs.•Small AuNPs can catalytically oxidize glucose like glucose oxidase.•AgNPs was used sensing platform for the product of H2O2.•l-cysteine assembled on metal nanoparticles can improve the sensitivity of glucose detection.We have coupled gold nanoparticles (AuNPs) with silver nanoparticles (AgNPs) to assemble a plasmonic sensing platform for colorimetric detection of glucose. In this system, small AuNPs (~ 4 nm) can act as glucose oxidase (GOD) mimic enzyme to catalytically oxidize glucose in the presence of oxygen, producing hydrogen peroxide, which dissolves AgNPs to lead the color changes. Glucose can be detected not only by naked eyes (from yellow to red) but also by spectrophotometer in the concentration range of 5–70 μM, with detection limit of 3 μM. More importantly, we found that l-cysteine added in the system can markedly improve the selectivity for the detection of glucose. The proposed method was used to application for the detection of glucose in human serum with satisfactory results. This system is simple and low cost without using any enzymes and organic chromogenic agents.

A sensitive spectrophotometric detection of glucose based on triangular silver nanoplates (Ag TNPs) coupled with gold nanoparticles (Au NPs) was carried out. Relying on the glucose oxidase (GOD) mimetics of small Au NPs, glucose was catalytically oxidized in the presence of oxygen. The product of H2O2 then induced the etching of Ag TNPs from triangular to round, which led to blue-shifting of the peak wavelength and a decrease of the absorbance. The color changes of the solution could be used for visual detection (from dark blue to light pink) of glucose by the naked eye. The blue-shift of the peak wavelength was linear proportional to the glucose concentration in the range from 0.5 to 20 μM with a detection limit of 0.3 μM. The high sensitivity of the method is due to the highly reactive tips and strong tip sharpness of Ag TNPs. Moreover, the effect of L-cysteine (Cys) assembled on metal nanomaterials was investigated in the system.

•The Au NPs onto ITO solid substrate was fabricated by self-assembly method and calcination.•The ITO/Au NPs/Cys probe can be used to plasmonic detection of Cu2+ ions.•The introduction of BSA can greatly improve the sensitivity of detection.•The plasmonic sensor was of high sensitivity and wide linear range.Gold nanoparticles (Au NPs) based plasmonic probe was developed for sensitive and selective detection of Cu2+ ion. The Au NPs were self-assembled on transparent indium tin oxide (ITO) film coated glass substrate using poly dimethyl diallyl ammonium chloride (PDDA) as a linker and then calcined at 400 °C to obtain pure Au NPs on ITO surface (ITO/Au NPs). The probe was fabricated by functionalizing l-cysteine (Cys) on to gold surface (ITO/Au NPs/Cys). The strong chelation of Cu2+ with Cys formed a stable Cys-Cu complex, and resulted in the red-shift of localized surface plasmon resonance (LSPR) peak of the Au NPs. The introduction of bovine serum albumin (BSA) as the second complexant could form complex of Cys-Cu-BAS and further markedly enhanced the red-shift of the LSPR peak. This plasmonic probe provided a highly sensitive and selective detection towards Cu2+ ions, with a wide linear detection range (10−11–10−5 M) over 6 orders of magnitude. The simple and cost-effective probe was successfully applied to the determination of Cu2+ in real samples.

•A reagentless, sensitive and selective optical sensor for detection of Hg(II) was developed.•Silver–gold nanocages were prepared on the transparent indium tin oxide coated glass surface.•The nanomaterials could act as optical sensing probe as well as reducing agent.•The plasmonic sensor could be used to detect mercury ions in field analysis.We demonstrate the utilization of silver/gold nanocages (Ag/Au NCs) deposited onto transparent indium tin oxide (ITO) film glass as the basis of a reagentless, simple and inexpensive mercury probe. The localized surface plasmon resonance (LSPR) peak wavelength was located at ∼800 nm. By utilizing the redox reaction between Hg2+ ions and Ag atoms that existed in Ag/Au NCs, the LSPR peak of Ag/Au NCs was blue-shifted. Thus, we develop an optical sensing probe for the detection of Hg2+ ions. The LSPR peak changes were lineally proportional to the concentration of Hg2+ ions over the range from 10 ppb to 0.5 ppm. The detection limit was ∼5 ppb. This plasmonic probe shows good selectivity and high sensitivity. The proposed optical probe is successfully applied to the sensing of Hg2+ in real samples.

We developed a localized surface plasmon resonance (LSPR)-based label-free optical biosensor for detection of salbutamol (Sal). Hollow gold nanoparticles (HGNs) which deposited on transparent indium tin oxide (ITO) film coated glass was used to sensing platform. Antibody against Sal was immobilized on HGN surface to recognize the target Sal molecules. Thus, the change of LSPR peak was proportional to the concentration of Sal in the solution. The experimental results demonstrated that the LSPR immunosensor possessed a good sensitivity and a high selectivity for Sal. The detection range for Sal was from 0.05 to 0.8 μg/mL with a correlation coefficient of 0.996. The biosensor was applied for the detection for Sal in spiked animal feed and pork liver samples, and the recoveries were in the range of 97–105 %. Therefore, it is expected that this approach may offer a new method in designing label-free LSPR immunosensor for detection of small molecules.

A facile and effective approach for the improvement of localized surface plasmon resonance (LSPR) biosensors based on silver-core and gold-shell nanoparticles (Ag@AuNPs) on a glass substrate was investigated. Silver nanoparticles (core) with thin gold shells on a transparent indium tin oxide (ITO) coated glass surface were prepared by sequential electrodeposition, and the influence of the thickness of the gold shell was systematically investigated. The experimental results indicate that the properties of an LSPR band of ultrathin (∼1.3 nm) gold-shell coated silver nanoparticles are very similar to those of silver nanoparticles alone. The refractive index (RI) sensitivities of the metal nanostructures are calculated as 123 and 220 nm/RIU for the silver cores (∼480 nm of LSPR peak) and Ag@AuNPs (∼503 nm of LSPR peak), respectively, on the ITO substrate. The RI sensitivity of Ag@AuNPs was significantly enhanced by coating the silver nanoparticles with an ultrathin gold shell. This core–shell platform was also applied to the fabrication of biosensors. Thus, this strategy can be used to construct inexpensive, stable, versatile, and sensitive LSPR biosensors.Keywords: electrodeposition; nanostructures; plasmonic sensors; silver core and gold shell;

A label-free and reagentless immunosensor based on localized surface plasmon resonance (LSPR) sensing was developed for the detection of salbutamol (Sal). Silver triangular nanoparticles deposited on transparent indium tin oxide film glass were used as the sensing platform. The immunosensor was prepared by direct adsorption of antibody against Sal on the surface of silver triangular nanoparticles. LSPR bands were obtained by a basic UV-vis spectrophotometer in the transmission mode. The analytical performance of the biosensor for the detection of Sal was examined. Under the optimized conditions, a calibration plot for Sal was obtained with a linear range between 0.02 μg mL−1 and 0.8 μg mL−1 (r = 0.991). The detection limit was 0.01 μg mL−1. The proposed biosensor has been used to detect the concentration of Sal in pig complex feed and pork liver samples. It is believed that the plasmonic immunosensor is simple, cost-effective and can be easily used for the detection of Sal in cases of its abuse in animal feed for field analysis.

In ammonia solution, binary gold-silver alloy films were directly deposited onto indium tin oxide (ITO) substrate surface by using electrochemical method with chloroauric acid and silver nitrate as precursors. Then high-surface-area nanoporous gold (NPG) film modified electrodes were fabricated by chemical dealloying of the less noble component silver from the surface. The modified electrodes were characterized. The electrode prepared can be successfully functionalized with self-assembled monolayers of L-cysteine, which can be applied to the sensitive and selective determination of Cu(II). Under the optimal conditions and 5 min of absorption time for Cu(II) ions, the cathodic current is linearly proportional to Cu(II) concentration ranged from 0.05 to 4 μM with a detection limit of 0.03 μM. The relative standard deviation is 4.3% for 1 μM Cu(II) solution. The established method was successfully applied for the determination of Cu(II) in water samples of environment.

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ∼700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.

The electrochemical and photoelectrochemical biosensors based on glucose oxidase (GOD) and ZnS nanoparticles modified indium tin oxide (ITO) electrode were investigated. The ZnS nanoparticles were electrodeposited directly on the surface of ITO electrode. The enzyme was immobilized on ZnS/ITO electrode surface by sol–gel method to fabricate glucose biosensor. GOD could electrocatalyze the reduction of dissolved oxygen, which resulted in a great increase of the reduction peak current. The reduction peak current decreased linearly with the addition of glucose, which could be used for glucose detection. Moreover, ZnS nanoparticles deposited on ITO electrode surface showed good photocurrent response under illumination. A photoelectrochemical biosensor for the detection of glucose was also developed by monitoring the decreases in the cathodic peak photocurrent. The results indicated that ZnS nanoparticles deposited on ITO substrate were a good candidate material for the immobilization of enzyme in glucose biosensor construction.Highlights► ZnS nanoparticles were electrodeposited directly on ITO surface. ► The direct electron transfer of GOD immobilized on ZnS surface was obtained. ► The enzyme electrode was used to the determination of glucose in the presence of oxygen. ► The response of photoelectrochemical biosensor towards glucose was more sensitive.

Au–ZnS core–shell nanostructures were grown onto the transparent indium tin oxide (ITO) thin film-coated glass surface by successive electrodeposition of Au and ZnS in cyclic voltammetry. The resulting hybrid nanostructures were characterized using scanning electron microscopy, X-ray diffraction, UV–vis spectroscopy, and electrochemical impedance spectroscopy. The glucose oxidase (GOD) was immobilized onto the surface of the Au–ZnS hybrid nanostructures in silica sol–gel network. Furthermore, the Au–ZnS nanostructures demonstrate an enhanced direct electron transfer between GOD and the electrode due to their unique chemical and electrocatalytic properties and their synergy effect. The analytical performance of the GOD-based electrode was improved greatly compared with that of ITO substrate modified by Au or ZnS nanostructures alone. The proposed enzyme electrode based on Au–ZnS hybrid nanomaterials displays high sensitivity and wide linear range in the determination of glucose. The Au–ZnS hybrid nanostructures have potential for “green chemistry” application in the fabrication of enzyme-based electrochemical biosensors.

Cysteine, an important amino acid, is an often-used self-assembly regent in the preparation of modified electrodes and biosensors. However, our understanding of the cysteine self-assembly process on gold surface is still not well understood. In this study, the electrochemical behavior of modified electrodes based on Cys self-assembly on gold surface was investigated. The cyclic voltammogram shows a pair of well-shaped redox peaks (0.14 V of anodic peak and −0.02 V of cathodic peak) at this modified electrode in phosphate buffer solution (pH 7.0). The electrochemical properties of the couple redox peaks were investigated systematically by cyclic voltammetry. The experiment results indicate that hydrous gold(I) oxide on the interfacial of the modified electrode exhibited good electrocatalytic activity of ascorbic acid oxidation. The formation mechanism of hydrous gold(I) oxide is also discussed. This oxide can participate in electrocatalytic processes as mediator to improve electrochemical oxidation of ascorbic acid.Highlights► We investigate electrochemical behavior at Au surface after self-assembled cysteine. ► The cyclic voltammogram shows a pair of well-shaped redox peaks. ► Hydrous gold(I) oxide is formed on the interfacial of the modified electrode. ► This oxide can participate in electrocatalytic processes as mediator.

A comparison of the electrochemical and photoelectrochemical behaviors of three biosensors, based on the use of Au, CdS, and ZnS nanoparticles–glucose oxidase (GOD) system, is discussed. All the nanoparticles were electrodeposited onto the indium tin oxide (ITO) thin film coated glass surface. GOD was then immobilized on the nanoparticles-modified electrodes surface with the sol–gel technique. The deposited nanoparticles on ITO electrodes were characterized by scanning electron microscopy, UV–vis spectroscopy and electrochemical impedance spectra. The direct electrochemistry of GOD, analytical performance of glucose calibration curves and the kinetic parameters of the enzyme reaction were compared for all the electrochemical biosensors. Furthermore, the current response of the quantum dots–GOD system biosensor can be increased after illumination. The electrochemical and photoelectrochemical biosensors based on ZnS nanostructures exhibited higher sensitivity than that of Au or CdS nanostructures. Considering ZnS is nontoxic to human and environment, the results suggest that ZnS nanoparticles–GOD system seems to be a promising platform for fabrication of novel electrochemical and photoelectrochemical biosensors.Highlights► The biosensors based on Au, CdS and ZnS nanoparticles–glucose oxidase system were constructed. ► Their electrochemical and photoelectrochemical behaviors were compared. ► The biosensor based on ZnS is more favorable in the fabrication of GOD-based electrochemical biosensors. ► This biosensor was also applied to determination of glucose in human serum samples and obtained satisfied results.

The refractive index sensitivity of gold@silver core-shell nanoparticles on indium tin oxide glass was investigated. The thin silver shell was electrodeposited on gold surface by controlling the applied potential and electrodeposition cycles. The refractive index sensitivity at gold@silver core-shell nanoparticles with ultrathin silver shell (~ 0.7 nm) to bulk refractive index was enhanced 76% than that of the Au nanoparticles (Au core) deposited on ITO substrate. This core-shell platform was also applied to detection of biomolecular interactions using the streptavidin-biotin assay. The strategy would reduce greatly the substrate effect of plasmonic sensors.Highlights► Thin silver shell was electrodeposited on surface of gold nanoparticles. ► This core-shell nanoparticles were supported by substrate as a sensor platform. ► The refractive index sensitivity was enhanced greatly at ultrathin Ag shell platform. ► This platform was also applied to detection of biomolecular interaction.

A simple one-step method for the electrochemical deposition of gold nanoparticles (GNPs) onto bare indium tin oxide film coated glass substrate without any template or surfactant was investigated. The effect of electrolysis conditions such as potential range, temperature, concentration and deposition cycles were examined. The connectivity of GNPs was analyzed by UV–Vis absorption spectroscopy and scanning electron microscopy. The nanoparticles were found to connect in pairs or to coalesce in larger numbers. The twin GNPs display a transverse and a longitudinal localized surface plasmon resonance (LSPR) band, which is similar to that of gold nanorods. The presence of longitudinal LSPR band correlates with high refractive index sensitivity. Conjugation of the twin-linked GNPs with albumin bovine serum–biotin was employed for the detection of streptavidin as a model based on the specific binding affinity in biotin/streptavidin pairs. The spectrophotometric sensor showed concentration-dependent binding for streptavidin.

We report on a new type of indium tin oxide (ITO) electrode for sensing ascorbic acid (AA). The ITO film was modified with gold-platinum alloy nanoparticles (Au-Pt NPs) functionalized with a self-assembled film of L-cysteine. The Au-Pt NPs were electrodeposited on the ITO film and characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction. A cyclic voltammetric study revealed that the electrode exhibits excellent electrocatalytic activity towards the oxidation of AA. The calibration plot for AA is linear over the concentration range from 2 to 400 μM with a correlation coefficient of 0.9991. The detection limit of AA is 1 μM.

Gold–platinum (Au–Pt) hybrid nanoparticles (Au–PtNPs) were successfully deposited on an indium tin oxide (ITO) surface using a direct electrochemical method. The resulting nanoparticles were characterized by scanning electron microscopy (SEM), UV–vis spectroscopy, X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and electrochemical methods. It was found that the size of the Au–PtNPs depends on the number of electrodeposition cycles. Au–PtNPs obtained by 20 electrodeposition cycles had a cauliflower-shaped structure with an average diameter of about 60 nm. These Au–PtNPs exhibited alloy properties. Electrochemical measurements showed that the charge transfer resistivity was significantly decreased for the Au–PtNPs/ITO electrode. Additionally, the Au–PtNPs displayed an electrocatalytic activity for nitrite oxidation and oxygen reduction. The Au–PtNPs/ITO electrodes reported herein could possibly be used as electrocatalysts and sensors.

A simple low-cost electrochemical approach has been used to directly deposit twin-linked gold nanoparticles (TGNPs) onto transparent indium tin oxide (ITO) coated film glass. The as-prepared TGNPs have a transverse localized surface plasmon resonance (LSPR) band at 540 nm and a longitudinal LSPR band at about 710 nm. The longitudinal LSPR band of TGNPs exhibits higher refractive index sensitivity (245 nm/RIU) than its transverse LSPR band. The resulting “clean” surface of the TGNPs is easy applied to the further modification. The subsequent bioconjugation of TGNP films with goat anti-mouse-immunoglobulin G (anti-m-IgG) is successfully employed for the detection of mouse-immunoglobulin G (m-IgG) in a model based on the specific binding affinity between the antigen and antibody. The spectrophotometric sensor shows concentration-dependent binding for m-IgG. This study reveals a simple and sensitive method to fabricate a label-free optical biosensor based on longitudinal LSPR band of TGNPs on ITO substrate.

Glucose oxidase (GOD) and horseradish peroxidase (HRP), entrapped alone and simultaneously in silica sol–gel (SG) network on gold nanoparticles (GNPs) modified indium tin oxide (ITO) electrode, were used to fabricate mono-enzyme GOD and bienzyme GOD–HRP glucose biosensors. The optimal conditions for the construction and the analytical performances of the biosensors were studied and compared. The optimal GOD concentration in mixed solution for the mono-enzyme and bienzyme glucose biosensors was about 250 U/mL, and the optimal GOD/HRP ratio was approximately 2 for the bienzyme glucose biosensors. Amperometric response of glucose was evaluated by holding the biosensors at 0.1 V (versus SCE). The linear ranges of detection for glucose were between 0.05–4.0 mmol/L and 0.02–3.2 mmol/L, respectively. The sensitivity of the bienzyme glucose biosensor was approximately 2.4-fold higher than that of mono-enzyme glucose biosensor. Both glucose biosensors showed rapid response, high selectivity and long-term stability.

Large size gold nanoparticles (GNPs) were directly deposited onto the indium tin oxide (ITO) glass surface by cyclic voltammetric method. The GNPs on ITO substrate were characterized by means of scanning electron microscopy (SEM), UV–vis spectroscopy and X-ray diffraction (XRD). The nucleation and growth steps were controllable in the GNPs deposition procedure. The addition of chloride ion in the electrolysis affected the size and density of GNPs on the ITO surface. The response of refractive index for various organic solvents was also investigated. The sensitivity of refractive index increased as GNPs became larger.

Gold nanoparticles (GNPs) were direct deposited onto the surface of indium tin oxide (ITO) electrode surface through cyclic voltammetric method. The GNPs-modified ITO electrodes were characterized by means of scanning electron microscopy (SEM), UV–vis spectroscopy and electrochemical methods. The quasi-spherical GNPs directly attached on the electrode surface with a relatively small size and a quite narrow distribution. Cyclic voltammetry and linear sweep voltammetry of glucose on the modified electrodes were performed. The GNPs-modified ITO electrodes showed high electrocatalytic reactivity towards glucose oxidation in 0.01 M NaOH and 0.05 M phosphate buffer solution (pH 7.4). The GNPs-modified transducer was successfully used for the amperometric sensing of glucose at alkaline and neutral solutions.

A novel disposable third-generation hydrogen peroxide (H2O2) biosensor based on horseradish peroxidase (HRP) immobilized on the gold nanoparticles (AuNPs) electrodeposited indium tin oxide (ITO) electrode is investigated. The AuNPs deposited on ITO electrode were characterized by UV–vis, SEM, and electrochemical methods. The AuNPs attached on the ITO electrode surface with quasi-spherical shape and the average size of diameters was about 25 nm with a quite symmetric distribution. The direct electron chemistry of HRP was realized, and the biosensor exhibited excellent performances for the reduction of H2O2. The amperometric response to H2O2 shows a linear relation in the range from 8.0 μmol L−1 to 3.0 mmol L−1 and a detection limit of 2 μmol L−1 (S/N = 3). The KMapp value of HRP immobilized on the electrode surface was found to be 0.4 mmol L−1. The biosensor indicates excellent reproducibility, high selectivity and long-term stability.

A novel disposable biosensor based on direct electron transfer of superoxide dismutase (SOD) was fabricated for the determination of superoxide anion. The biosensor was constructed by electrodeposition of gold nanoparticles (GNPs) on the indium tin oxide (ITO) electrode and then immobilization of SOD in silica sol–gel (SG) network in the presence of cysteine on GNPs/ITO modified electrode surface. The distribution of GNPs on ITO electrode surface was examined by scanning electron microscopy (SEM). The immobilized SOD exhibited high catalytical activity towards superoxide anion. Parameters affecting the performance of the biosensor were also investigated. A linear calibration curve was obtained over the range from 0.08 to 0.64 μM with a correlation coefficient of 0.9937. The resulted biosensors were demonstrated to possess striking analytical properties for superoxide anion determination, such as high sensitivity, good accuracy, and long-term stability. It provides a promising platform for the fabrication of disposable biosensors.

A novel third-generation biosensor for hydrogen peroxide (H2O2) has been constructed based on horseradish peroxidase (HRP) immobilized by the sol–gel (SG) technology on carbon nanotube (CNT)-modified electrode. CNT has good promotion effects on the direct electron transfer between HRP and the electrode surface and the SG network provides a biocompatible microenvironment for enzyme. The immobilized HRP retained its bioelectrocatalytic activity for the reduction of hydrogen peroxide and can respond to the change of concentration of H2O2 rapidly. The heterogeneous electron transfer rate constant was evaluated to be 2.8 ± 0.4 s−1. The amperometric response to H2O2 shows a linear relation in the range from 0.5 to 300 μmol l−1 and a detection limit of 0.1 μmol l−1 (S/N = 3). The KMapp value of HRP immobilized on the electrode surface was found to be 1.35 mmol l−1. The biosensor exhibited high sensitivity, rapid response and excellent long-term stability.

A label-free and reagentless immunosensor based on localized surface plasmon resonance (LSPR) sensing was developed for the detection of salbutamol (Sal). Silver triangular nanoparticles deposited on transparent indium tin oxide film glass were used as the sensing platform. The immunosensor was prepared by direct adsorption of antibody against Sal on the surface of silver triangular nanoparticles. LSPR bands were obtained by a basic UV-vis spectrophotometer in the transmission mode. The analytical performance of the biosensor for the detection of Sal was examined. Under the optimized conditions, a calibration plot for Sal was obtained with a linear range between 0.02 μg mL−1 and 0.8 μg mL−1 (r = 0.991). The detection limit was 0.01 μg mL−1. The proposed biosensor has been used to detect the concentration of Sal in pig complex feed and pork liver samples. It is believed that the plasmonic immunosensor is simple, cost-effective and can be easily used for the detection of Sal in cases of its abuse in animal feed for field analysis.