RESUMO
The present study reports a direct approach to generate efficient hot spots using a nonresonant molecule bound to the inner part or hot spots that can increase the SERS sensitivity to obtain spectra of one and a few molecules. The 4-Mercaptobenzoic acid (4-MBA) adsorbate, connected to the Ag surface by a thiolate-Ag bonding, was able to trigger a self-assembly process of AgNP, which occurred by cooperative hydrogen bonds between the carboxylic groups of 4-MBA located in different nanoparticles when the pH was adjusted to 4. The self-assembly structure was characterized by UV-Vis spectroscopy and SERS (Surface Enhancement Raman Scattering), and DFT-based calculation of the model complex [AgNP-(4-MBA)2-AgNP] was employed to improve the understanding of the self-assembled complex formation through the comparison of calculated and experimental SERS spectra. The SERS signal of 4-MBA on AgNP above the pKa of the carboxyl group was not observed below 1 × 10-6 mol L-1 in any condition. Additionally, the SERS spectra of 4-bromobezenothiol (4-BrBT) at 5.0 × 10-7 mol L-1 had no bands assignable to 4-BrBT, reinforcing the hypothesis that the SERS intensification for 4-MBA in low concentrations (below 1 × 10-6 mol L-1) is due to the hydrogen bonding triggered self-assembly of AgNP below 4-MBA pKa. The average SERS of the 4-MBA in low surface coverage shows a mixture of structures, such as protonated and deprotonated 4-MBA, as well as a small amount of benzenethiol coming from decarboxylation of part of 4-MBA molecules. The few molecules SERS detection of 4-MBA was demonstrated experimentally and the experimental results were associated with a greater number of hot spots formed, being befitting with the generalized Mie theory simulations.
RESUMO
Acute myocardial infarction (AMI) is nowadays the leading death cause worldwide. For that reason, the early diagnosis of AMI is of central importance to reduce the risk of death. In this sense, aptamer-based sensors for surface-enhanced Raman spectroscopy (SERS aptasensors) emerged as an interesting alternative for future high-performance diagnostic tools. SERS aptasensors combine the fast, precise, and sensitive nature of SERS measurements with the selectivity of aptamers for specific biological targets. Herein, we report an efficient SERS aptasensor for the detection of cardiac troponin I (cTnI), a gold-standard biomarker for AMI. Our SERS platform comprises a magnetite core with an intermediate silica shell, and a flower-shaped silver layer (Fe3O4@SiO2@Ag) to confer excellent plasmonic properties and ease of collection by magnetism. The branched silver structure combined with magnetic aggregation offers a high near-field amplification to superior SERS performance. Additionally, a tailored DNA aptamer with high specificity for cTnI was anchored to the silver surface to produce the aptasensor with increased sensing capability towards cTnI. With our SERS aptasensor, a cTnI concentration as low as 10 ng ml-1 (10-11 mol l-1) could be detected. This value is ten times lower than the upper threshold of the typical concentration range of cTnI of AMI patients. Hence, our SERS aptasensor holds great promise to be explored in AMI diagnosis.
