The authors describe a Surface enhanced Raman spectroscopy (SERS)-based method for the detection of gaseous toluene at different temperature regimes using 3D ruffled silver SERS substrates and a commercially available handheld Raman system equipped with a 785 nm laser. The 3D silver SERS substrates were synthesized via electroless deposition of silver on the ruffled sandpaper and HF-etched silicon wafers. The morphological characterization of the silver SERS substrates was carried out by atomic force microscopy and scanning electron microscopy. UV-Vis spectroscopy absorption spectra of the silver nanostructures showed plasmonic peaks at 522 nm and 731 nm. Toluene vapors were collected with a syringe at ambient temperature and at 100 °C, while SERS detection was always performed at room temperature. Toluene detection was based on the measurement of the Raman bands at 787 cm?1 and 1003 cm?1 (in the fingerprint region). The method allow gaseous toluene to be detected at its vapor concentrations of 522 ppm (mg/L), 261 ppm (mg/L) and 26 ppm (mg/L).
Graphical abstract Schematic presentation of an original method for the detection of toluene vapors by SERS technique. The collection of toluene vapors was carried out at room and at high temperatures. The vapors were transferred to methanol by bubbling. The SERS measurements were carried out at room temperature.
A new method is described for the determination of the pesticide λ-cyhalothrin (LC). It combines SERS detection with molecular imprinting and largely improves selectivity. A multilayer surface imprinted nanocomposite was synthesized in two steps on a nanostructure of type SiO2@rGO@Ag acting as a substrates. Firstly, the surface of the SiO2@rGO@Ag composite was modified with self-polymerized dopamine. Secondly, surface-initiated polymerization was carried out to prepare a molecularly imprinted polymer (MIP) using LC as the template. The use of this SiO2@rGO@Ag-MIP allows for excellent SERS based detection and has high selectivity for LC. The Raman intensity and LC concentration present perfect linear relationship between 10?5 to 10?9 mol L?1 and the detection limit is 3.8×10?10 mol L?1. All the procedures are conducted in aqueous or ethanol solution.
Graphical abstract Schematic of a new method for determination of the pesticide λ-cyhalothrin. It combines SERS detection with molecular imprinting and largely improves selectivity. A multilayer surface imprinted nanocomposite was synthesized in two steps on a nanostructure of type SiO2@rGO@Ag acting as a substrates.
A composite consisting of carbon nanotubes (CNT) and copper nanoparticles (CuNPs) was prepared by a chemical reduction method, and its structure characterized by scanning electron microscopy, transmission electron microscopy energy dispersive spectroscopy and FT-IR spectrometry. The hybrid composite was deposited on the surface of a disposable gold electrode that was manufactured from a commercial digital versatile gold disc by a drop casting method. The electrochemical properties of the modified electrode were investigated by cyclic voltammetry and differential pulse voltammetry. The sensor showed an excellent electrocatalytic activity towards oxidation of paracetamol (PA). The calibration plot (with current typically measured at 0.41 V vs. Ag/AgCl) is linear in the 0.5 to 80 μM concentration range, and the detection limit is as low as 10 nM. The sensor was successfully applied to the determination of PA in spiked water and tablet samples where it gave recoveries ranging between 95.25 and 100.5 %.
Graphical abstract Carbon nanotubes (CNT) -copper nanoparticles (CuNPs) hybrid composite was synthesized by a facile method then the nanohybrid was used as a modifier for the DVD gold electrode for improving its performance toward paracetamol electrooxidation. Cyclic voltammetry and differential pulse voltammetry were used for characterization and determination of paracetamol, respectively.
A SERS-based aptasensor for ochratoxin A (OTA) is described. It is making use of Fe3O4@Au magnetic nanoparticles (MGNPs) and of Au@Ag nanoprobes modified with the Raman reporter 5,5-dithiobis-(2-nitrobenzoic acid; DTNB). Au-DTNB@Ag NPs were modified with the OTA aptamer (aptamer-GSNPs) and used as Raman signal probes. The SERS peak of DTNB at 1331 cm?1 was used for quantitative analysis. MGNPs modified with cDNA (cDNA-MGNPs) were used as capture probes and reinforced substrates. When the Au-DTNB@Ag-Fe3O4@Au complexes are formed through oligonucleotide hybridization, the Raman signal intensity of the Raman probe is significantly enhanced. If the OTA concentration in samples increases, more Raman signal probes (aptamer-GSNPs) will dissociate from the cDNA-MGNPs because more OTA aptamer is bound by OTA. This leads to a lower Raman signal after magnetic separation. Under the optimal conditions, the detection limit for OTA is 0.48 pg·mL?1 based on 3σ criterion. This is attributed to the multiple Raman signal enhancement and the good performance of the OTA aptamer. The good recovery and accuracy of the assay was confirmed by evaluating spiked samples of wine and coffee.
Graphical abstract Schematic of an aptamer based SERS assay for OTA by integrating Fe3O4@AuNPs (MGNPs) with Au-DTNB@Ag NPs with multiple signal enhancement. Aptamer modified Au-DTNB@Ag NPs are used as Raman probes, and MGNPs modified with cDNA are used as capture probes and reinforced substrates.
Platinum nanoparticles (PtNPs) were uniformly grown on the surface of gold nanorods (AuNRs) by a laser irradiation procedure. Transmission electron microscopy confirmed that the PtNPs are uniformly grown on the surface of the AuNRs. The formation of PtNPs on the AuNRs leads to a red-shift of the absorption maximum from 734 nm to 766 nm. In addition, the efficiency of surface enhanced Raman scattering (SERS) is increased, but the photothermal conversion efficiency is decreased compared to pure AuNRs. The result indicates that electron transfer occurs between gold and platinum. The peroxidase mimicking effect of PtNPs, AuNRs and Au/Pt NRs by catalyzing the oxidation of colorless 3,3’,5,5’-tetramethylbenzidine (TMB) to blue oxidized 3,3’,5,5’-tetramethylbenzidine (oxTMB; a quinone) in the presence of H2O2. The catalytic activity of Au/Pt NRs is higher than that of sole AuNRs or PtNPs by factors of 4.2 and 2.1, respectively. Thus, Au/Pt NRs have been used for the detection of peroxide and the limit of detection is 0.04 μM. This work provides an approach to integrate the peroxidase mimicking effect with SERS enhancement for potential application in detection.
Graphical abstract A schematic diagram for the laser-induced growth of Au/Pt NRs and the colorimetric determination of hydrogen peroxide concentration with their peroxidase mimicking properties. The limit of detection is 0.04 μM based on the use of Au/Pt NRs as a catalyst.
It is known that gold nanoparticles (AuNPs) possess peroxidase-like activity. They can catalyze the oxidation of 3,3,5,5-tetramethylbenzidine by H2O2 which leads to a color change from red to blue. It is shown here that the peroxidase-like activity of AuNPs can be inhibited by passivating its surface passivation with a ssDNA aptamer against sulfadimethoxine. If, however, the target molecule (sulfadimethoxine) is present, the aptamer is desorbed from the AuNPs surface, and this results in the reactivation of the catalytic property of the AuNPs. The color change of the solution (from purple to blue) is related to the analyte concentration, and this can be judged visually or by UV-visible absorptiometry at 650 nm. The assay, under optimized conditions, has a detection limit of 10 ng·mL?1 of sulfadimethoxine, and the calibration plot is linear over a rather wide concentration range (0.01–1000 μg·mL?1). The assay can be performed within <15 min, is sensitive, and therefore is well suited for fast screening in food analysis. Conceivably, it can be extended to many other small analytes for which aptamers are available.
Graphical abstract Aptamer based photometric assay for sulfadimethoxine(SDM) based on the inhibition and reactivation of the peroxidase-like activity of gold nanoparticles (AuNPs) was performed with a rather wide linear range (0.01–1000 μg?mL?1) and low detection limit of 10 ng?mL?1.
The article describes the preparation of chitosan-coated hemoglobin (Hb-CS) microcapsules by (a) preparing a CaCO3 precipitate containing Hb, (b) crosslinking Hb with glutaraldehye, (c) coating the particles with chitosan, and (d) preparing Hb-CS microcapsules by removing the CaCO3 template with a solution of disodium EDTA. The morphology and electrochemical properties of the Hb-CS microcapsules were investigated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. An oxygen sensor was obtained by immobilizing the Hb-CS microcapsules on the surface of a glassy carbon electrode (GCE) first modified with gold nanoparticles. The application of Hb-CS microcapsules facilitates electron transfer on the surface of GCE and warrants the integrity and biological activity of Hb. The oxygen sensor, operated best at a working voltage of ?0.335 V (vs. SCE), displays a low limit of detection (30 nM). The Hb-CS microcapsules also are shown to release loaded oxygen to an anaerobic aqueous environment within 300 min.
The authors describe a composite material prepared from carbon nanohorns and poly(2-aminopyridine) that was obtained by electrochemical polymerization of 2-aminopyridine on carbon nanohorns. The material was used to modify a glassy carbon electrode (GCE) to obtain a sensor for non-enzymatic determination of hydrogen peroxide. The modified GCE was characterized by cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry. The modified electrode is shown to display excellent electrical conductivity and catalytic activity towards hydrogen peroxide, mainly due to the large specific surface area of carbon nanohorns, the good electron charge transfer properties resulting from the use of poly(2-aminopyridine), and their synergistic effect. The response of the modified GCE (best operated at a working potential of ?0.45 V vs. SCE) to H2O2 is linear in the 0.05 to 8 mM concentration range. The limits of detection (LOD) and quantitation (LOQ) are 3.6 μM and 12.4 μM, respectively. The electrode is selective, stable and reproducible, this making it a promising tool for non-enzymatic determination of hydrogen peroxide.
The paper describes a sensitive method for simultaneous sensing of morphine (MOR) and diclofenac (DCF). The surface of a MgFe2O4/graphite paste electrode was modified with multi-walled carbon nanotubes, and the resulting sensor was characterized by cyclic voltammetry, differential pulse voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The electrode showed an efficient synergistic effect in term of oxidation of DCF and MOR, with sharp oxidation peaks occurring at +0.370 and 0.540 V (vs Ag/AgCl) at pH 7.0. The calibration plot for MOR is linear in the 50 nM to 920 μM concentration range, and the detection limit is 10 nM (at a signal-to-noise ratio of 3). The respective data for DCF are 100 nM to 580 μM, with a 60 nM LOD. The sensor was applied to the determination of MOR and DCF in spiked serum and urine samples, with recoveries ranging between 91.4 and 100.7 %.
Graphical abstract A sensitive method for simultaneous sensing of morphine (MOR) and diclofenac (DCF) is described. The surface of MgFe2O4/graphite paste electrode was modified with multi-walled carbon nanotubes, and the resulting sensor showed an efficient synergistic effect in terms of oxidation of DCF and MOR. The calibration plot for MOR is linear in the 50 nM to 920 μM concentration range, and the detection limit is 10 nM. The respective data for DCF are 100 nM to 580 μM, with a 60 nM LOD.
The authors describe a method for the colorimetric determination of unamplified microRNA. It is based on the use of citrate-capped gold nanoparticles (AuNPs) and, alternatively, a microRNA-probe hybrid or a magnetically extracted microRNA that serve as stabilizers against the salt-induced aggregation of AuNPs. The absorbance ratios A525/A625 of the reacted AuNP solutions were used to quantify the amount of microRNA. The assay works in the range of 5–25 pmol microRNA. The lower limit of detection (LOD) is 10 pmol. The performance of the method was tested by detection of microRNA-210-3p in totally extracted urinary microRNA from normal, benign, and bladder cancer subjects. The sensitivity and specificity for qualitative detection of urinary microRNA-210-3p using the assay are 74% and 88% respectively, which is consistent with real time PCR based assays. The assay was applied to the determination of specific microRNA by using its specific oligo targeter or following magnetic isolation of the desired microRNA. The method is simple, cost-efficient, has a short turn-around time and requires minimal equipment and personnel.
Graphical abstract Schematic of the two detection schemes: In the first approach, matched microRNA hybridizes with its specific probe to stabilize gold nanoparticles (AuNPs) against salt induced aggregation and to leave the red color of the AuNPs unchanged. In the second one, microRNA extracted via magnetic nanoparticles (MNP) stabilizes AuNPs against aggregation.
A novel photoelectrochemical (PEC) aptasensor with graphitic-phase carbon nitride quantum dots (g-C3N4; QDs) and reduced graphene oxide (rGO) was fabricated. The g-C3N4 QDs possess enhanced emission quantum yield (with an emission peak at 450 nm), improved charge separation ability and effective optical absorption, while rGO has excellent electron transfer capability. Altogether, this results in improved PEC performance. The method is making use of an aptamer against sulfadimethoxine (SDM) that was immobilized on electrode through π stacking interaction. Changes of the photocurrent occur because SDM as a photogenerated hole acceptor can further accelerate the separation of photoexcited carriers. Under optimized conditions and at an applied potential of +0.2 V, the aptasensor has a linear response in the 0.5 nM to 80 nM SDM concentration range, with a 0.1 nM detection limit (at S/N =?3). The method was successfully applied to the analysis of SDM in tap, lake and waste water samples.
Graphical abstract Graphitic-phase carbon nitride (g-C3N4) quantum dots (QDs) and reduced graphene oxide (rGO) were used to modify fluorine-doped SnO2 (FTO) electrodes for use in a photoelectrochemical (PEC) aptasensor. SDM oxidized by the hole on valance band (VB) of g-C3N4 QDs promote the separation of electron in the conductive band (CB), which made the changes of photocurrent signal.
Molecular imprints (MIPs) composed of CdTe quantum dots and tetracycline-imprinted polymethacrylates were synthesized on the surface of flexible plastic polyimide sheets from methacrylic acid, allyl mercaptan, and ethylene glycol dimethacrylate. The amount of aqueous CdTe solution, ethanol diluent, the reaction time and temperature of the radical-initiated polymerization were optimized. The MIP-QD composites were then copolymerized on the methacrylated polyimides. The resulting films were then immersed into a supersonic water-ethanol bath to strip off the tetracycline (Tc) templates. The fluorescence quenching intensity measured at 565 nm under 315 nm excitation before and after dropping a Tc sample on the film was used to optimize the processes. The calibration plot is linear in the 70 μM to 2.2 mM Tc concentration range at pH 7.5, and the LOD is 8.8 μM. The relative standard deviation is 8.2% (for n = 10). The method was applied to the determination of Tc in spiked samples of bovine serum albumin and fetal bovine serum and gave recoveries of 98% (at 200 μg?mL?1) and 97% (at 1.0 ppt), respectively.
Graphical abstract Flexible polyimide substrates were chemically modified with methacrylate groups for anchoring the composites (MIP-QD) of tetracycline-imprinted polymers and CdTe quantum dots. The fluorescence of the integrated substrates is quenched by tetracycline but not by methacycline and steroids.
The authors describe a method for DNA target recognition and signal amplification that is based on the target-induced formation of a three way junction. The subsequent assembly of two DNA probes releases the inhibitory strand and triggers a downstream strand displacement amplification. This causes the formation of a G-rich single sequence that binds to a hemin monomer with its peroxidase-mimicking properties. The resulting peroxidase (POx) activity is quantified by using H2O2 and TMB as the substrate. In the presence of an inhibitor, in contrast, the POx-like activity is strongly reduced. This forms the basis for a highly sensitive DNA assay. It has a 0.8 pM detection limit when operated at a wavelength of 450 nm and was applied to the isothermal determination of target DNA with high selectivity.
Graphical abstract Schematic of the assay: Introduction of target results in the formation of a three way junction. The subsequent assembly of two probes releases the inhibitory strand and triggers a downstream strand displacement amplification, generating amount of G-rich single sequence which causes peroxidase-mimicking activity on binding to a hemin monomer.
The authors describe an aptamer-based detection scheme that is based on untemplated nucleic acid elongation and the use of copper nanoparticles (CuNPs) as a fluorescent probe. An aptamer without any other auxiliary sequence and label is required only which makes the method rather convenient. Under the catalysis of terminal deoxynucleotidyl transferase (TdT), the single-stranded aptamer is elongated without template. By using dTTPs as the substrate, long linear poly T can be produced, and these can act as templates for the synthesis of CuNPs which display red (617 nm) fluorescence under 349 nm photoexcitation. In the presence of the analyte, the TdT-catalyzed production of poly T is blocked, and this results in suppressed fluorescence. The strategy was successfully applied to the determination of the proteins thrombin and vascular endothelial growth factor 165. Only three steps are involved in the whole assay. This aptamer-based assay is believed to have a wide scope in that it may be applied to the analysis of many other proteins if the corresponding aptamers are available.
Graphical abstract A versatile aptamer-based method for the determination of thrombin and VEGF165 is introduced. It is based on TdT catalyzed nucleic acid elongation and poly T templated formation of fluorescent copper nanoparticles.
The authors describe an upconversion nanoparticle-based (UCNP–based) fluorometric method for ultrasensitive and selective detection of Cu2+. The UCNPs show a strong emission band at 550 nm under near-infrared excitation at 980 nm. The principle of the strategy is that gold nanoparticles (AuNP) can quench the fluorescence of UCNP. In contrast, the addition of L-cysteine (Cys) can induce the aggregation of AuNP, resulting in a fluorescence recovery of the UCNPs. On addition of Cu2+, it oxidizes Cys to cystine and is reduced to Cu+. The Cu+ thusformed can be oxidized cyclically to Cu2+ by dissolved O2, which catalyzes and recycles the whole reaction. Thus, the aggregation of AuNP is inhibited and the fluorescence recovered by Cys is quenched. Under the optimal condition, the quenching efficiency shows a good linear response to the concentrations of Cu2+ in the 0.4–40 nM range. The limit of detection is 0.16 nM, which is 5 orders of magnitude lower than the U.S. Environmental Protection Agency limit for Cu2+ in drinking water (20 μM). The method has been further applied to monitor Cu2+ levels in real samples. The results of detection are well consistent with those obtained by atomic absorption spectroscopy.
Graphical abstract Gold nanoparticles (AuNP) as a high efficient fluorescence quenching reagent of upconversion nanoparticles (UCNP) were used in a fluorometric method for detection of Cu2+ based on a cyclic catalytic oxidation amplification strategy.
A magnetic glassy carbon electrode (mGCE) was modified with a ternary composite prepared from Prussian blue (PB), magnetite (Fe3O4) nanoparticles, and reduced graphene oxide (rGO) in order to obtain an amperometric sensor for hydrazine. The utilization of Fe3O4 facilitates the magnetic immobilization and separation of sensing material, while the use of rGO enhances sensitivity. The surface coverage and the stability of the PB on the modified electrode were considerably improved. The electro-oxidative response to hydrazine was investigated with this modified mGCE using cyclic voltammetry and amperometric. The sensor, typically operated at a voltage of 0.2 V (vs. SCE), displays superior response hydrazine, with a response time of 4 s, a sensitivity of 97.73 μA μM?1 cm?2 and a 13.7 nM detection limit.
Graphical abstract A magnetic glassy carbon electrode was modified with a ternary composite prepared from Prussian blue, magnetite nanoparticles, and reduced graphene oxide to obtain a selective amperometric sensor for dissolved hydrazine.
An efficient approach is demonstrated for preparing particles consisting of a silver core and a shell of molecularly imprinted polymer (Ag@MIP). The MIP is prepared by using bisphenol A (BPA) as the template and 4-vinylpyridine as the functional monomer. The Ag@MIP fulfills a dual function in that the silver core acts as a SERS substrate, while the MIP allows for selective recognition of BPA. The Ag@MIP is characterized by scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, thermogravimetric analysis and Raman spectroscopy. The Raman intensity of Ag@MIP is higher than that of bare silver microspheres. The detection limit for BPA is as low as 10?9 mol·L?1.
Graphical abstract Schematic illustration of the preparation of silver microspheres coated with a molecularly imprinted polymer (Ag@MIPs) for detecting bisphenol A (BPA) by surface enhanced Raman scattering (SERS).
The authors describe a sensitive surface-enhanced Raman scattering (SERS)-based aptasensor for the detection of the food pathogen Vibrio parahaemolyticus. Nanostructures consisting of Fe3O4@Au particles wrapped with graphene oxide (GO) were used both as SERS substrates and separation tools. A first aptamer (apt 1) was immobilized on the Fe3O4@Au/GO nanostructures to act as a capture probe via the affinity binding of aptamer and V. parahaemolyticus. A second aptamer (apt-2) was modified with the Raman reporter molecule TAMRA to act as a SERS sensing probes that binds to the target the same way as the Fe3O4@Au/GO-apt 1. The sandwich formed between Fe3O4@Au/GO-apt 1/V. parahaemolyticus and apt 2-TAMRA can be separated with the aid of a magnet. The concentration of V. parahaemolyticus can be quantified by measurement of the SERS intensity of TAMRA. Under optimal conditions, the signal is linearly related to the V. parahaemolyticus concentration in the range between 1.4 × 102 to 1.4 × 106 cfu·mL?1, with a detection limit of 14 cfu·mL?1. Recoveries ranging from 98.5% to 105% are found when analyzing spiked salmon samples. In our perception, the assay described here is a useful tool for quantitation of V. parahaemolyticus in real samples.
Graphical abstract GO wrapped Fe3O4@Au nanostructures were synthesized as the substrate and modified with with a first aptamer (apt 1) to capture V. parahaemolyticus. TAMRA labelled aptamer 2 was then used as signal probe. The V. parahaemolyticus concentrations are closely related to the Raman intensity of TAMRA.
The authors report on a robust method for the synthesis of gold nanorods (AuNRs) with tunable dimensions and longitudinal surface plasmon resonance. The method relies on seed-mediated particle growth in the presence of benzalkonium chloride (BAC) in place of the widely used surfactant cetyltrimethyl ammonium bromide (CTAB). Uniform AuNRs were obtained by particle growth in solution, and BAC is found to stabilize the AuNRs for >1 year. The SERS activity of the resulting AuNRs is essentially identical to that of CTAB-protected nanorods. The SERS activity of the BAC protected nanorods was applied to the quantitative analysis of potato virus X (PVX). The calibration plot for PVX is linear in the 10 to 750 ng?mL?1 concentration range, and the detection limit is 2.2 ng?mL?1.
Graphical abstract SERS-active gold nanorods (AuNRs) have been prepared by using benzalkonium chloride as stabilization agent. Effects of chemical parameters on AuNRs have been explored and AuNRs were used in quantitative analysis of potato virus X (PVX).
The authors have developed a straightforward colorimetric method for the rapid determination of lysozyme by using citrate-capped gold nanoparticles (AuNPs) with different particle sizes but without any further surface modification. It is found that AuNPs (15 nm i.d.) undergo aggregation in the presence of lysozyme owing to the high abundance of amino groups in lysozyme. Aggregation leads to a color change of the solution from red over purple to bluish-purple that can be detected visually or by photometry. The limit of detection is 20 nM. We further show that the use of AuNPs with 5 and 15 nm i.d. can improve the sensitivity of the assay compared to using bare AuNPs by adding HAuCl4 and NH2OH to the solution which induces the growth of AuNPs and leads to a smaller interparticle space between AuNPs. This gives rise to differently colored solutions, with color transitions from red to bluish-purple to colorless. The LODs are 0.1 nM for both the 5-nm and 15-nm AuNPs. Compared to the LOD when using a solution of 15-nm AuNPs and without chloroauric acid and hydroxylamine, the LOD (0.1 nM) is lower by a factor of 200. The method is sensitive, specific, and does not require sophisticated equipment. Its feasibility was demonstrated by analyzing lysozyme in samples of egg white.
Graphical abstract We utilized 4 kinds of gold AuNPs with different particle sizes (5, 15, 30, and 50 nm) as colorimetric probes for lysozyme analysis.
