The authors describe a selective and sensitive colorimetric method for determination of dopamine (DA) in serum. It is based on the protection of silver nanoprisms (AgNPRs) from being etched by chloride ion in the presence of DA. In the absence of DA, the shape of the AgNPRs is changed from triangle nanoplates to round nanodisk due to etching by chloride. This is accompanied by a stepwise color change from blue via purple and red to yellow. It is found that DA is strongly adsorbed on the surface of AgNPRs and thereby acts as a protective agent. As a result, etching by chloride is prevented and the color changes do not occur. This finding ease exploited to design a method for optical quantification of DA with either bare eyes or UV-vis spectrophotometry. The wavelength shift of the in-plane dipole surface plasmon band of the AgNPRs is linearly related to the DA concentration in the range from 0.5 to 100 nM. The detection limit is 0.16 nM (at an S/N ratio of 3) which is lower than that of most existing methods. Uric acid, ascorbic acid and other coexisting species do not interfere. The sensor is reproducible and stable and was applied to the determination of DA in spiked serum samples where it gave recoveries that ranged from 97.4 to 104.3%.

The authors describe a colorimetric assay for the determination of As(III) in aqueous solution using citrate capped gold nanoparticles (AuNPs) which, in the presence of As(III), undergo aggregation due to the interaction of citrate ion with As(III). This results in an easily detectable color change from wine-red to blue. The ratio of the absorbances at 661 and 519 nm is linearly related to the As(III) concentration in the 4 to 100 ppb range, with a detection limit as low as 1.8 ppb (at 3σ) which is below the guideline value of 10 ppb. The method is rather simple in that it does not require the surface of the AuNPs to be modified. It was successfully applied to the determination of As(III) in spiked drinking water where it gave adequate recoveries.

•Ternary nanocomposites PAMAM-AuNPs/SnO2/graphenes nanosheets (GNs) were obtained with a facile method.•The obtained nanomaterial was used to construct the electrochemical sensor for the detection of dopamine (DA) in the presence of ascorbic acid.•The electrochemical sensor could be applied to the determination of DA in real samples.The nanocomposite of polyamidoamine (PAMAM) modified gold nanoparticles (pAuNPs)/SnO2/graphene sheets (GNs) was prepared by a new method. That is, the well dispersed AuNPs were prepared in the presence of PAMAM and resembled on the surface of 5,10,15,20-tetrakis (4-sulonatophenyl) porphyrin (H4TPPS2–) modified graphite oxide (GO) by electrostatic interaction. Then the well-distributed SnO2 was obtained from SnCl2 with the aid of GO and GO was reduced to graphene by hydrazine hydrate. The synthesized nanocomposite was characterized by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction analysis (XRD). Cyclic voltammetry (CV) experiment demonstrated that the nanocomposite exhibited excellent electrochemical performance for the detection of dopamine (DA). Then the quantitative detection of DA was carried out by differential pulse voltammetry (DPV) in the presence of ascorbic acid (AA). A linear range of 0.03–10 μM was obtained with a correlation coefficient of 0.9970. The detection limit was as low as 8 nM (S/N = 3). Moreover, the proposed method was applied for the determination of DA in real serum samples.A facile method for the preparation of polyamidoamine (PAMAM)–gold nanoparticles (AuNPs)/SnO2/graphenes nanosheets (GNs) nanocomposites is presented. Highly sensitive and selective detection of dopamine could be realized by the proposed method.Download high-res image (127KB)Download full-size image

•A dual-amplified electrochemical immunosensor based on Au/ZnO was fabricated.•C12N3 acted as the surfactants of Au/ZnO/RGO.•C18N3 acted as the reductant agents of Au@ZnO composite.•XRD, Raman, FT-IR, UV-Vis, SEM have been used for characterization.•Fabricated immunosensor showed excellent sensitivity and specificity.Herein, a dual signal amplification strategy was employed in fabricating ultrasensitive electrochemical immunosensor for alpha fetoprotein (AFP) detection, which was realized by utilizing of ZnO nanorods/Au nanopaticles hybridized reduced graphene nanosheet (Au/ZnO/RGO) and horseradish-peroxidase (HRP) bioconjugated detection antibody (Ab2) functionalized Au@ZnO (Ab2/HRP-Au@ZnO). During the fabrication of the immunosensor, a new kind of multiple-head surfactants CxN3 with different alkyl chain length played important roles such as acting as the surfactants of Au/ZnO/RGO and the reductant agents of Au@ZnO composite. Due to the good adsorption property and large surface area of Au/ZnO/RGO, plenty of the capture antibodies (Ab1) were immobilized on the electrode surface, and trace AFP was sensitively monitored. Furthermore, Ab2/HRP-Au@ZnO exhibited high affinity interaction with AFP through “sandwich” immunoreactions, along with the peroxidase-like catalytic activity of Au@ZnO, leading to a further enhancement in the sensitivity of the proposed immunosensor. The successful synthesis of the nanomaterials was characterized through a serious of techniques including Raman, XRD, FT-IR, SEM and UV–vis. Under the optimal conditions, two linear ranges of 0.02–10,000 and 10,000–100,000 pg mL-1 AFP with a lower detection limit of 0.01  pg mL-1 (S/N=3) was obtained. Especially, the proposed AFP immunosensor can be applied to detect human serum samples with satisfactory results, indicating a potential application in clinical monitoring of tumor biomarkers.

In the present work, polydopamine (PDA) modified reduced graphene oxide (pRGO)/SnO2/Au nanoparticles (NPs) were synthesized. SnO2/AuNPs were prepared through the redox reaction between reductive stannous(II) ions and oxidative auric(III) ions without any other reagents, and then the as-prepared graphene oxide (GO) was reduced and stabilized with the help of DA followed by decorating with SnO2/AuNPs. The morphology and structure of the hybrid nanomaterials were characterized by Raman spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction analysis. Then we constructed electrochemical sensors based on the hybrid nanomaterials for the determination of dopamine (DA). Cyclic voltammetry (CV) results showed that the hybrid nanomaterials had better sensing performance than pRGO/AuNPs and pRGO. Differential pulse voltammetry (DPV) showed that the separation of oxidation peak potentials for ascorbic acid (AA)–DA, uric acid (UA)–DA and UA–AA was about 200 mV, 100 mV and 300 mV, respectively. The linear range for the detection of DA was from 0.008 μM to 20 μM with a coefficient of 0.9986. The detection limit was 5 nM (S/N = 3). These indicate the selective and sensitive detection of DA. Finally, the constructed sensors were also applied to detect DA in human real samples.

A colorimetric detection method for melamine based on 1,4-dithiothreitol (DTT) functionalized gold nanoparticles (DTT–AuNPs) is reported. Hydrogen-bonding interaction between DTT and melamine resulted in the aggregation of AuNPs, which consequently resulted in a color change of the AuNPs from wine red to blue and a red-shift of the surface plasmon resonance (SPR) peak of the DTT–AuNPs. The concentration of melamine could be determined by the naked eye or using a UV-vis spectrometer. FT-IR and TEM were used to investigate the modification of DTT–AuNPs. The results showed that the absorption ratio (A680/A520) was linear with the logarithm of melamine concentration in the range of 8.0 × 10−8 M to 6.0 × 10−7 M and 6.0 × 10−7 M to 1.5 × 10−6 M with linear coefficients of 0.993 and 0.999, respectively. The detection limit (S/N = 3) of the proposed method was 2.4 × 10−8 M, which is considerably lower than the safety limit. Coexisting substances did not affect the determination of melamine. Furthermore, the proposed method was applied for the detection of trace amounts of melamine in real milk samples with recoveries of 96–103%.

•A novel ternary graphene/SnO2/Au was synthesized.•High sensitive and label-free electrochemical immunosensor for detection of α-fetoprotein was developed.•The proposed electrochemical immunosensor was successfully applied to the determination of AFP in serum samples.A label-free electrochemical immunosensor for sensitive detection of α-fetoprotein (AFP) was developed based on graphene/SnO2/Au nanocomposite. The graphene/SnO2/Au nanocomposite modified glassy carbon electrode was used to immobilize α-fetoprotein antibody (anti-AFP) and to construct the immunosensor. Results demonstrated that the peak currents of [Ru(NH3)6]3+ decreased due to the interaction between antibody and antigen on the modified electrode. Thus, a label-free immunosensor for the detection of AFP was realized by monitoring the peak current change of [Ru(NH3)6]3+. The factors influencing the performance of the immunosensor were investigated in details. Under optimal conditions, the peak currents obtained by DPV decreased linearly with the increasing AFP concentrations in the range from 0.02 to 50 ng mL−1 with a linear coefficient of 0.9959. This electrochemical immunoassay has a low detection limit of 0.01 ng mL−1 (S/N=3) and was successfully applied to the determination of AFP in serum samples.

A novel one-pot synthesis of graphene nanosheet/SnO2 nanoparticle hybrid nanocomposites (GN/SnO2) was realized by using graphene oxide nanosheets (GONs) functionalized with sodium dodecyl sulfonate and SnCl2 as the starting materials. The morphology and structure of the synthesized SDS-GN/SnO2 nanocomposites were characterized by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction analysis. It was found that SnO2 nanoparticles were homogeneously distributed on the graphene nanosheets. The electrochemical behavior of dopamine (DA) at the SDS-GN/SnO2 nanoparticle modified electrode was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results showed that the modified electrode exhibited excellent electrocatalytic activity towards the electrochemical oxidation of DA. The separation of the oxidation peak potentials for ascorbic acid (AA)-DA, uric acid (UA)-DA and UA-AA obtained by DPV is about 132 mV, 128 mV and 260 mV, respectively, which allows selective and sensitive detection of DA in the presence of AA and UA. The anodic peak currents were linear with the concentration of DA in the range from 1.0 × 10−7 to 1.0 × 10−5 M with a coefficient of 0.9980. The detection limit was 80 nM (S/N = 3). The proposed method could be applied for the determination of DA in real human urine samples.

•A colorimetric strategy for sensing four antibiotics was proposed.•Hydrogen bond between pyrocatechol violet and antibiotics played an important role.•This strategy can be applied in the pretreated liquid milk products.Antibiotics residues in foods are very harmful to human beings. Determination of antibiotics residues relies largely on the availability of adequate analytical techniques. Currently, there is an urgent need for on site and real time detection of antibiotics in food. In this work, a novel one step synthesis of gold nanoparticles (AuNPs) was proposed using pyrocatechol violet (PCV) as a reducer agent. Highly sensitive and selective colorimetric detection of four antibiotics kanamycin mono sulfate (KA), neomycin sulfate (NE), streptomycin sulfate (ST) and bleomycin sulfate (BL) was realized during the formation of AuNPs. PCV has OH groups and these antibiotics have OH, NH2, NH groups, so there may be some special hydrogen-bonding interactions between PCV and these antibiotics. Therefore, the presence of KA, NE, ST and BL would influence the synthesis of AuNPs, then the color and state of AuNPs would change, which could be observed with the naked eye or a UV–vis spectrophotometer. Results showed that A670 was linear with the logarithm of KA concentration in the range from 1.0 × 10−8 to 5.0 × 10−7 M and 5.0 × 10−7 to 5.5 × 10−5 M. The detection limit of KA was 1.0 × 10−9 M (S/N = 3). The coexisting substances including 1.0 × 10−5 M phenylalanine, alanine, glycerol, glucose, Mg2+, Ca2+, Na+, K+, CO32−, SO42−, NO3−, Cl− and Br− did not affect the determination of 1.0 × 10−7 M antibiotics. In particular, the proposed method could be applied successfully to the detection of antibiotics in the pretreated liquid milk products.The presence of antibiotic substrates kanamycin mono sulfate, neomycin sulfate, streptomycin sulfate and bleomycin sulfate influence the synthesis of AuNPs, including the color and state of AuNPs, which could be used to detect antibiotic substrates with the naked eye or a UV–vis spectrophotometer.

A highly sensitive and selective method was presented for colorimetric determination of melamine using gold nanoparticles (AuNPs). AuNPs were synthesized using ellagic acid (EA) as reducer in distilled water at room temperature without adding nanoparticle seeds and stabilizing agent. Melamine could interact with EA through a strong hydrogen-bonding interaction leading to weakening of the EA reducing ability, thus the formation of AuNPs was interrupted and the color of the solution changed from red to pale yellow. The concentration of melamine could be quantified visually or using a UV-vis spectrometer in the wide range from 1.6 × 10−8 M to 1.6 × 10−4 M with a correlation coefficient of 0.9961. The detection limit (3σ) was as low as 1.6 × 10−9 M. This sensor was simple, inexpensive and highly sensitive and could be successfully utilized to detect melamine in pretreated liquid milk products with high recoveries from 93% to 106%. Besides, it exhibited excellent selectivity against other interferences.

•A novel graphene biosensor for Hg2+ based on T–Hg2+–T structure is realized.•The single-strand DNA was grafted on the RGO surface via Michael addition reaction.•A detection limit of 5.0×10−9 M Hg2+ was obtained (S/N=3).•The proposed method was used to analyze Hg2+ in river water.A novel electrochemical biosensor for sensitive and selective detection of mercury (II) ions (Hg2+) based on a DNA grafted graphene is proposed. Graphene oxide (GO) was reduced by dopamine, and then the single-strand probe DNA modified at the 5′-end with an alkylamino modifier (NH2-ssDNA) was grafted on the reduced graphene oxide (RGO) surface via Michael addition reaction. In the presence of Hg2+, the target DNA with four thymine–thymine (T–T) mismatches would hybridize with the probe DNA on the glassy carbon electrode (GCE) through T–Hg2+–T coordination chemistry. The hybridization of the two oligonucleotides leads to the increase in the peak currents of [Ru(NH3)6]3+, which could be used for electrochemical sensing of Hg2+. The difference in the value of the peak currents of [Ru(NH3)6]3+ before and after DNA hybridization was linear with the concentration of Hg2+ in the range from 8.0×10−9 to 1.0×10−7 M with a linear coefficiency of 0.996. The detection limit was 5.0×10−9 M (S/N=3). The proposed electrochemical biosensor is rapid, convenient and low-cost for effective sensing of Hg2+. Particularly, the proposed method was applied successfully to the determination of Hg2+ in real environmental samples.

A novel one-pot synthesis of graphene nanosheet/SnO2 nanoparticle hybrid nanocomposites (GN/SnO2) was realized by using graphene oxide nanosheets (GONs) functionalized with sodium dodecyl sulfonate and SnCl2 as the starting materials. The morphology and structure of the synthesized SDS-GN/SnO2 nanocomposites were characterized by Raman spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction analysis. It was found that SnO2 nanoparticles were homogeneously distributed on the graphene nanosheets. The electrochemical behavior of dopamine (DA) at the SDS-GN/SnO2 nanoparticle modified electrode was studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results showed that the modified electrode exhibited excellent electrocatalytic activity towards the electrochemical oxidation of DA. The separation of the oxidation peak potentials for ascorbic acid (AA)-DA, uric acid (UA)-DA and UA-AA obtained by DPV is about 132 mV, 128 mV and 260 mV, respectively, which allows selective and sensitive detection of DA in the presence of AA and UA. The anodic peak currents were linear with the concentration of DA in the range from 1.0 × 10−7 to 1.0 × 10−5 M with a coefficient of 0.9980. The detection limit was 80 nM (S/N = 3). The proposed method could be applied for the determination of DA in real human urine samples.

A highly sensitive and selective method was presented for colorimetric determination of melamine using gold nanoparticles (AuNPs). AuNPs were synthesized using ellagic acid (EA) as reducer in distilled water at room temperature without adding nanoparticle seeds and stabilizing agent. Melamine could interact with EA through a strong hydrogen-bonding interaction leading to weakening of the EA reducing ability, thus the formation of AuNPs was interrupted and the color of the solution changed from red to pale yellow. The concentration of melamine could be quantified visually or using a UV-vis spectrometer in the wide range from 1.6 × 10−8 M to 1.6 × 10−4 M with a correlation coefficient of 0.9961. The detection limit (3σ) was as low as 1.6 × 10−9 M. This sensor was simple, inexpensive and highly sensitive and could be successfully utilized to detect melamine in pretreated liquid milk products with high recoveries from 93% to 106%. Besides, it exhibited excellent selectivity against other interferences.