SERS Determination of Oxidative Stress Markers in Saliva Using Substrates with Silver Nanoparticle-Decorated Silicon Nanowires

Abstract

Glutathione and malondialdehyde are two compounds commonly used to evaluate the oxidative stress status of an organism. Although their determination is usually performed in blood serum, saliva is gaining ground as the biological fluid of choice for oxidative stress determination at the point of need. For this purpose, surface-enhanced Raman spectroscopy (SERS), which is a highly sensitive method for the detection of biomolecules, could offer additional advantages regarding the analysis of biological fluids at the point of need. In this work, silicon nanowires decorated with silver nanoparticles made by metal-assisted chemical etching were evaluated as substrates for the SERS determination of glutathione and malondialdehyde in water and saliva. In particular, glutathione was determined by monitoring the reduction in the Raman signal obtained from substrates modified with crystal violet upon incubation with aqueous glutathione solutions. On the other hand, malondialdehyde was detected after a reaction with thiobarbituric acid to produce a derivative with a strong Raman signal. The detection limits achieved after optimization of several assay parameters were 50 and 3.2 nM for aqueous solutions of glutathione and malondialdehyde, respectively. In artificial saliva, however, the detection limits were 2.0 and 0.32 μM for glutathione and malondialdehyde, respectively, which are, nonetheless, adequate for the determination of these two markers in saliva.

Superoxide Dismutase Detection on Silver Nanostructured Substrates through Surface-Enhanced Spectroscopic Techniques

“…The substrates utilized in this work were fabricated with the MACE method, and included SiNWs decorated with either Ag dendrites or Ag nanoparticle aggregates. The substrates with the Ag dendrites were selected for the SEF measurements while those with the Ag aggregates were employed for the SERS measurements, based on previous findings that indicated Ag dendrites were more effective in SEF, whereas aggregates exhibited a higher signal and an enhanced reproducibility in SERS measurements [40,41].…”

“…The silver substrates were fabricated with a simple metal-assisted chemical etching (MACE) procedure, as previously described [24,40]. Our study focused on evaluating the performance of two distinct substrate types: one containing Ag dendrite-decorated Si nanowires (SiNWs) (type A) and the other containing Ag aggregate-decorated Si nanowires (type B), both created onto p-type (100) oriented monocrystalline silicon wafers.…”

“…For this reason, two distinct configurations of silver nanoparticles were fabricated on silicon surfaces with a simple metal-assisted chemical etching process and evaluated as substrates for the detection of SOD. Our team has already successfully employed these substrates for the determination of two oxidative stress markers, namely glutathione and malondialdehyde, and for the ovarian cancer markers Ca125 and HE4, indicating the potential of these metallic nanostructures in the detection of biological molecules at low concentrations [40,41]. The protocol for SOD determination was based on a competitive immunoassay configuration, and two different signal enhancement approaches were investigated.…”

Cancer Marker Immunosensing through Surface-Enhanced Photoluminescence on Nanostructured Silver Substrates

“…To this end, the aim of this work was to take advantage of nanostructured silicon surfaces decorated with silver nanoparticles as substrates for immunochemical detection through the photoluminescence of two cancer markers, namely carbohydrate antigen 125 (CA125) and human epididymis protein 4 (HE4). The substrates were fabricated by employing metal-assisted chemical etching (MACE), which is a simple procedure utilized to create, in a single step, nanowire structures on a silicon wafer with Ag dendrite structures on top of them [19][20][21] (Figure 1). The substrates can be used as prepared or treated in an acidic solution to remove the Ag dendrites and create aggregates of Ag nanoparticles by post-immersion in the etching/deposition solution (Figure 1).…”

“…The substrates can be used as prepared or treated in an acidic solution to remove the Ag dendrites and create aggregates of Ag nanoparticles by post-immersion in the etching/deposition solution (Figure 1). Both types of substrates have been exploited for the detection of biomolecules through surface-enhanced Raman spectroscopy [19,20], and their potential for enhancement of photoluminescence has been demonstrated using directly adsorbed fluorescent labels [21]. In this work, the two types of nanostructured surfaces were evaluated with respect to the immunochemical detection of CA125 and HE4, two ovarian cancer markers, aiming at the development of a system that could determine both analytes simultaneously in human serum samples.…”