Elias de Barros Santos

Fast detection of paracetamol on a gold nanoparticle–chitosan substrate by SERS

A fast method for detecting pharmaceutical drugs, such as paracetamol, by surface-enhanced Raman spectroscopy (SERS) using a gold nanoparticle substrate was studied. Gold nanoparticles were synthesized using chitosan (AuNP–chitosan) as a reductant and capping agent and subsequently deposited on glass slides as a thin film. The SERS performance of AuNP–chitosan films was evaluated using methylene blue (MB, 10−6 mol L−1) as a SERS probe molecule. The method is based on drop-drying an analyte solution (paracetamol, 10−3 mol L−1) onto a substrate surface and subsequently analyzing by Raman spectroscopy. The spectra were obtained in 10 seconds with two accumulations and exhibit a high signal-to-noise ratio. This preliminary study supports the AuNP–chitosan substrate as a SERS sensor, for a convenient analytical method for detection of paracetamol and other pharmaceutical drug molecules.

Silver nanoparticles: green synthesis, self-assembled nanostructures and their application as SERS substrates

In this paper, silver nanoparticles were synthesized using citrus peel extracts from species such as Citrus sinensis (orange fruit, AgNP-Ora), Citrus reticulata (tangerine fruit, AgNP-Tan), and Citrus aurantifolia (lemon fruit, AgNP-Lem). The absorption spectra of the AgNP-Ora and AgNP-Tan colloids show localized surface plasmon resonance (LSPR) bands at 445 and 423 nm, respectively. For the same synthesis method, a low intensity LSPR band for AgNP-Lem is observed, indicating a low yield of this reaction. However, the transmission electron microscopy images show that the colloid prepared using lemon extract also presents AgNP larger than 5 nm (5–55 nm), which exhibit plasmonic properties. The synthesized silver nanoparticles are spherical in shape and are highly crystalline, and they were self-assembled on NH2-modified glass slides, forming AgNP aggregate substrates for all the three prepared silver colloids. We also explored the SERS activity of the AgNP substrates using 10−6 mol L−1 solutions of 4-aminobenzenethiol, rhodamine 6G, and methylene blue as Raman probe molecules. It was possible to detect with a high signal-to-noise ratio the SERS spectral pattern of all probe molecules on AgNP substrates. This simple, low cost, and greener method for synthesizing silver nanoparticles may be valuable in future studies about SERS sensor development and can be extended to catalytic applications.

Using Gold Nanoparticles as Passive Sampler for Indoor Monitoring of Gaseous Elemental Mercury

This study reports a functional passive sampler for monitoring gaseous elemental mercury (GEM). The material consists of gold nanoparticles (AuNP) deposited forming a film on thiolmodified glass slides. AuNP colloid was synthesized using sodium citrate, resulting in a size of 24.2 ± 0.8 nm, as determined by transmission electron microscopy analysis. For the indoor test, AuNP films were placed into Petri plates. One part of the lot was kept exposed to the laboratory’s atmosphere and the other part was stored in closed plate (control). The films were removed from both plates along a month and analyzed using a direct mercury analyzer. A linear relationship between Hg retention and exposure time was obtained until the 23 day, which allow to calculate a GEM retention rate of 2.2 ng g day; the maximum retention capacity was around 50 ng g. This study provides an efficient method for indoor monitoring of GEM using AuNP films as passive sampler.

Intercalated 4-Aminobenzenethiol between Au and Ag Nanoparticles: Effects of Concentration and Nanoparticles Neighborhood on its SERS Response

In this work is reported the investigation of the 4-aminobenzenethiol (4-ABT) surface-enhanced Raman scattering (SERS) response for different 4-ABT concentrations as well as when this molecule is intercalated between Ag and Au nanoparticles (AgNP and AuNP). SERS substrates consisting of the individual AgNP or AuNP and bi-layer AgNP/4-ABT/AuNP or AuNP/4-ABT/AgNP were prepared on glass slides. It was observed a decreasing trend of the bands intensity ratios b2/a1 as a function of the 4-ABT solutions concentration. Since the b2 modes are enhanced through the charge transfer (CT) mechanism, this result indicates that this mechanism is less effective for low concentration of 4-ABT. Also, a dependence between a1 and b2-type bands intensities could be identified for the samples where 4-ABT molecules were intercalated between AgNP and AuNP. A significantly increase in the SERS intensities of the a1 mode around 1074 cm-1 and b2 mode around 1142 cm-1 is observed when AgNP are present in the samples. This result indicates that AgNP promote amplifications of the Raman signal thorough the CT and electromagnetic (EM) mechanisms, whereas for the AuNP the SERS effect is related manly to EM mechanism. These results show that the 4-ABT SERS response can provide more information than the SERS substrate activity. Its SERS signal pattern is rich in information, allowing investigations about the local where the molecule is, details about the substrate surface, and also on the mechanisms present in the system under study.

Proof of concept for a passive sampler for monitoring of gaseous elemental mercury in artisanal gold mining

Mercury emissions from artisanal gold mining operations occurring in roughly 80 developing countries are a major workplace health hazard for millions of people as well as the largest contributor to global mercury pollution. There are no portable, cheap, and rapid methods able to inform workers or health practitioners of mercury exposure on site in remote locations. In this work, a proof of concept for a miniaturized mercury sampler, prepared by the direct reduction of gold into the porous nanostructures of Vycor glass (PVG), is introduced. Mercury retention on the PVG/Au sampler induces significant color changes, due to the formation of Au-Hg amalgam that affects the surface plasmon resonance characteristics of the material. The color change can potentially be quantified by the analysis of pictures obtained with a cell phone camera rapidly and onsite. Laboratory experiments showed the viability of using PVG/Au as passive sampler for monitoring of Hg°. PVG/Au samplers were then deployed in an artisanal and small-scale gold mining (ASGM) operations in Burkina Faso and it was able to indicate personal mercury exposures. The amount of mercury quantified in the samplers for all miners was higher than the current personal exposure limit set by the US Occupational Safety & Health Administration (OSHA).

Facile synthesis of the dendritic structure of silver nanoparticles–chitosan and its application as an effective SERS substrate

The dendritic structure with high concentration of monodisperse silver nanoparticles into the chitosan matrix (D-AgNP@Chit) was prepared via a reduction process using sodium borohydride. The D-AgNP@Chit substrates were prepared by drop-drying AgNP@Chit colloidal solution on amine modified glass slides. The preparation conditions and SERS performance of the D-AgNP@Chit substrates were systematically investigated. Such a D-AgNP@Chit SERS substrate exhibits an enhancement factor of 2.1 × 106 and a detection limit of 10−12 mol L−1 for 4-aminobenzenethiol (4-ABT) could be achieved. Moreover, the SERS spectra repeatability of 4-ABT without any interference, from the support background, could be maintained over the dendritic structures. The fast time of detection and repeatability of the SERS measurements qualify the D-AgNP@Chit substrate as an effective SERS sensor for molecular analysis.