Essy Kouadio Fodjo

Low temperature synthesis and SERS application of silver molybdenum oxides

AgxMoyOz (Ag/α-MoO3, Ag/h-MoO3, Ag/β-MoO3, Ag2Mo2O7, and Ag2MoO4) nanomaterials are synthesized at relatively low temperature (80 °C) and with a short hydrothermal treatment time (3 h) using glucose (Glc) as a catalyst. Contrary to previous studies, it is possible to synthesize one of the AgxMoyOz binaries at 80 °C by altering the pH of the reaction media in the presence of glucose. In addition, further experiments indicate that Ag/h-MoO3, Ag2Mo2O7 and Ag/β-MoO3 can be converted to Ag/α-MoO3 which can also be synthesized from a reaction involving ammonium heptamolybdate tetrahydrate and AgNO3 at room temperature using cysteamine in both methods. Characterization with UV-Vis, XRD and Raman spectroscopy reveals high purity of as-synthesized product. These synthesized products are finally used to fabricate SERS substrates by coating a silicon wafer with a mixture of silver nanoparticles (Ag NPs) and AgxMoyOz (Si@Ag/AgxMoyOz). The results show a good sensitivity and stability. In addition, it is also found that the SERS activity of Si@Ag/AgxMoyOz is higher than that of Si@Ag.

Cu@Ag/β-AgVO3 as a SERS substrate for the trace level detection of carbamate pesticides

A Surface Enhanced Raman Scattering (SERS) substrate based on Ag/β-AgVO3 nanobelts deposited on copper foil (Cu@Ag/β-AgVO3) was used for the detection of carbamate pesticides: carbofuran, carbaryl, isoprocarb and propoxur. Cu@Ag/β-AgVO3 has shown an excellent SERS activity for carbamate pesticides compared to silver nanoparticles (AgNPs). Under optimized conditions, detection limits of 2.5 pM, 10 pM, 50 pM and 75 pM were obtained for carbaryl, carbofuran, isoprocarb and propoxur respectively, suggesting that Cu@Ag/β-AgVO3 is a good candidate for use as a SERS substrate for the trace level detection of these pesticides.

Simultaneous preconcentration and ultrasensitive on-site SERS detection of polycyclic aromatic hydrocarbons in seawater using hexanethiol-modified silver decorated graphene nanomaterials

Synthesis of high-quality metal nanoparticle (NP) decorated graphene (GN) with uniform and controllable nanostructures has attracted considerable attention due to its excellent electronic structure and optical properties. Herein, we develop a facile and robust process for tailoring hexanethiol (SH)-modified Ag NP-decorated graphene nanomaterials (Ag/GN-SH) by coupling in situ reduction and the covalent assembly technique. The as-prepared Ag/GN-SH combines adequate Raman “hot spots” among the high-density Ag NPs and the excellent adsorption performance of GN, making it an excellent surface-enhanced Raman scattering (SERS) substrate for highly selective and sensitive detection of PAHs. Moreover, the Ag/GN-SH can be used as a solid-phase extraction (SPE) adsorbent for qualitative and quantitative detection of PAHs in seawater from their discriminant SERS peaks. The Ag/GN-SH-based SPE-SERS is effective over a wide range of concentrations (0.1 nM to 0.5 mM) for PAHs, with tens of pM detection limits [signal to noise ratio (S/N) = 3], and the detection concentration deviation is less than 15% between the SPE-SERS and GC-MS. This study not only offers a new method for on-site removal of PAHs but also provides a strategy for simultaneous multiplexed detection of PAHs.

Chitosan-based adsorption and freeze deproteinization: Improved extraction and purification of synthetic colorants from protein-rich food samples

A freeze method for deproteinization coupling with the chitosan purification process was developed for the determination of 8 synthetic food colorants in protein-rich samples. The solvents for extraction and different methods for deproteinization were examined and selected. Chitosan was employed for the purification after deproteinization, and further compared with the traditional polyamine purification method. Determination of the purified extract was conducted through the separation using high performance liquid chromatography and detection by multi-wavelength mode. Under the optimum conditions, the method showed good linearity between 0.6 and 10mg/kg, for the 8 synthetic colorants, and the limit of detection was between 0.1 and 0.4 mg/kg as was defined when the ratio of signal to noise was three. The recoveries of the spiked samples were found to be between 83% and 91%. The intra-day precision and inter-day precision was estimated to be 3-10% and 6-12%, respectively. The developed method could be applied to deproteinization and clean-up for pretreatment of protein-rich samples.

Thioanisole induced size-selective fragmentation of gold nanoparticles

In this work, we report the features of size-selective fragmentation of gold nanoparticles (AuNPs) by thioanisole. Large particles are observed as prone to easier fragmentation than the smaller ones, the key-role being played by the effect of surface charge and surface coverage.

Highly sensitive “turn-on” fluorescence probe for the detection of sparfloxacin in human serum using silica-functionalized CdTe quantum dots

In this study, an environment-friendly hydrothermal synthesis of hydrophilic silica-functionalized CdTe quantum dots (SQDs) has been developed by treatment of tetraethyl orthosilicate (TEOS) in the presence of CdTe QDs. The proposed SQDs show favorable photoluminescence (PL) at the peak wavelength of 620 nm with a quantum yield of approximately 32%, and exhibit excellent selectivity and sensitivity toward sparfloxacin (SPFX) in the range from 0.05 × 10−6 to 200 × 10−6 mol L−1. The detection limit of SPFX is as low as 0.035 × 10−6 mol L−1. The results show that the remarkably enhanced PL intensity of SQDs can be attributed to the hydrogen-bond interactions between the SQDs and SPFX. Furthermore, the amount of SPFX in the human serum samples detected by the present method and HPLC are in good agreement, indicating that the proposed SQDs can serve as a powerful tool for biological analysis.

Correction to: Loop-mediated isothermal amplification for visual detection of Vibrio parahaemolyticus using gold nanoparticles

The published version of this article, unfortunately, contained error. Modifications have been made to the Abstract, Introduction, Results and discussion, and Acknowledgements section. The original article has been corrected.

Temperature Effect Study on Growth and Survival of Pathogenic Vibrio parahaemolyticus in Jinjiang Oyster (Crassostrea rivularis) with Rapid Count Method

The growth of Vibrio parahaemolyticus (V. parahaemolyticus) in oysters during postharvest storage increases the possibility of its infection in humans. In this work, to investigate the growth or survival profiles in different media, pathogenic V. parahaemolyticus in APW, Jinjiang oyster (JO, Crassostrea rivularis) slurry, and live JO were studied under different temperatures. All the strain populations were counted through our double-layer agar plate (DLAP) method. In APW, the pathogenic V. parahaemolyticus showed continuous growth under 15, 25, and 35°C, while a decline in behavior was displayed under 5°C. The similar survival trend of pathogenic V. parahaemolyticus in JO slurry and live JO was observed under 5, 25, and 35°C, except the delayed growth or decline profile compared to APW. Under 15°C, they displayed decline and growth profile in JO slurry and live JO, respectively. These results indicate the different sensitivity of pathogenic V. parahaemolyticus in these matrices to temperature variation. Furthermore, nonpathogenic V. parahaemolyticus displayed little difference in survival profiles when inoculated in live JO under corresponding temperatures. The results indicate that inhibition or promotion effect could be regulated under different storage temperature for both pathogenic and nonpathogenic strains. Besides, the DLAP method showed the obvious quickness and efficiency during the bacteria count.

Rapid and sensitive on-site detection of pesticide residues in fruits and vegetables using screen-printed paper-based SERS swabs

Ideal SERS substrates and rapid sampling are crucial for on-site detection of pesticide residues. In this work, silver nanoparticles and graphene oxide were successfully prepared on cellulose paper (Ag NPs/GO paper) using a screen printing technique. The screen printing ink containing graphene oxide and silver nanoparticles was orderly printed onto cellulose paper substrates by controlling the printing cycles to fabricate controllable SERS substrates in large batches for the detection of pesticide residues. It was found that for up to four cycles, the SERS effect increased with the number of Ag NP printing cycles and no obvious change was observed. The prepared Ag NPs/GO paper may exhibit a high enrichment ability due to the π–π stacking and electrostatic interactions of GO toward pesticide molecules. The results suggest that the high density of Ag NPs/GO on the prepared paper is enough for ultrasensitive SERS detection of thiram, thiabendazole and methyl parathion in complex surfaces with a low limit of detection of 0.26 ng cm−2, 28 ng cm−2, and 7.4 ng cm−2, which are much lower than the maximal residue limits in fruit prescribed by the U.S. Environmental Protection Agency. Thus, the sensitive SERS detection of pesticide residues based on Ag NPs/GO paper offers great potential for its practical application in environmental analysis and food safety.

Electrocatalytic Oxidation of Tris(2-carboxyethyl)phosphine at Pyrroloquinoline Quinone Modified Carbon Nanotube through Single Nanoparticle Collision

Inspired by the addition-elimination catalytic mechanism of natural pyrroloquinoline quinone (PQQ) containing proteins, PQQ-modified hybrid nanomaterials have been increasingly developed recently as biomimetic heterogeneous electrocatalysts. However, up until now, no existing electrochemical approach was able to assess the intrinsic catalytic activity of PQQ sites, impeding the design of efficient PQQ-based electrocatalysts. Herein, in this work, we introduced a new method to calculate the turnover frequency (TOF) of any individual PQQ functional group for electrocatalytic oxidation of tris(2-carboxyethyl)phosphine (TCEP), through the study of single PQQ-decorated carbon nanotube (CNT) collisions at a carbon fiber ultramicroelectrode by chronoamperometry. The core advantage of this approach is being able to resolve the number of PQQ catalytic sites grafted on each individual CNT, so that the charge of any CNT collision event can be accurately translated into the intrinsic activity of the respective PQQ functional groups. The resulting collision-induced current responses clearly showed that the functionalization of CNTs with PQQ could indeed enhance its catalytic performance by 3 times, reaching a TOF value of 133 s-1 at 1.0 V vs Ag/AgCl. Such a single CNT collision technique, which is proposed for the first time in this work, can open up a new avenue for studying the intrinsic (electro)catalytic performance at a molecular level.