Digital Protocol for Chemical Analysis at Ultralow Concentrations by Surface-Enhanced Raman Scattering

Abstract

Single molecule surface-enhanced Raman spectroscopy (SM-SERS) has the potential to revolutionize quantitative analysis at ultralow concentrations (less than 1 nM). However, there are no established protocols to generalize the application of this technique in analytical chemistry. Here, a protocol for quantification at ultralow concentrations using SM-SERS is proposed. The approach aims to take advantage of the stochastic nature of the single-molecule regime to achieved lower limits of quantification (LOQ). Two emerging contaminants commonly found in aquatic environments, enrofloxacin (ENRO) and ciprofloxacin (CIPRO), were chosen as nonresonant molecular probes. The methodology involves a multivariate resolution curve fitting known as non-negative matrix factorization with alternating least-squares algorithm (NMF-ALS) to solve spectral overlaps. The key element of the quantification is to realize that, under SM-SERS conditions, the Raman intensity generated by a molecule adsorbed on a "hotspot" can be digitalized. Therefore, the number of SERS event counts (rather than SERS intensities) was shown to be proportional to the solution concentration. This allowed the determination of both ENRO and CIPRO with high accuracy and precision even at ultralow concentrations regime. The LOQ for both ENRO and CIPRO were achieved at 2.8 pM. The digital SERS protocol, suggested here, is a roadmap for the implementation of SM-SERS as a routine tool for quantification at ultralow concentrations.

Surface Enhanced Raman Spectroscopy for Single Molecule Protein Detection

“…1b). This is an expected trend since the low number of deposited molecules on the SERS substrate decreases the probability for a hotspot to be occupied 20 . Representative spectra obtained by mapping substrates with high and low loadings of BSA-TR and low loading of Glycine-TR are shown in Fig.…”

“…If the amount of an analyte on the SERS substrate is low, selected spots only generate Raman signal. Should such situation be observed using the automatic mapping approach, it is indicative of SER spectra generated from a single hot spot 20 . For comparison, we collected SER spectra from substrates loaded with different amounts of BSA-TR adduct.…”

Biossensores Plasmônicos Baseados Em Espalhamento Raman Intensificado Por Superfície Utilizando Nanobastões De Ouroǁ

“…Parte da heterogeneidade no sinal SERS está associada à presença de regiões no AuNRs-chip que tem uma baixa densidade de AuNRs assim como regiões com uma aglomeração considerável das nanopartículas, o que resulta em uma heterogeneidade intrínseca do substrato. Para garantir que esse tipo de heterogeneidade intrínseca não interfira na determinação da qualidade do biossensor, será utilizada a proposta de procedimento de Albuquerque e colaboradores, 43 que envolve a construção um mapeamento digital da medida SERS adaptada ao presente substrato, que não envolve estudos em concentrações de moléculas individuais. No presente trabalho, os valores de intensidade SERS acima de um valor limite observados no mapeamento foram considerados como eventos positivos, e a eles foi atribuído o valor 1, e abaixo desse limite foram considerados eventos nulos, atribuídos valores 0, resultando em um mapeamento de eventos positivos e negativos.…”

Characterisation of estrogen receptor alpha (ERα) expression in breast cancer cells and effect of drug treatment using targeted nanoparticles and SERS

“…[22][23][24] Brolo et al focused on quantitatively detecting single molecules based on SERS signal intensity. 25 Additionally, Schultz et al showed that SERS can be used to distinguish specific to non-specific nanoparticle receptor binding. 26 Therefore, it is clear that SERS is a powerful method for the detection of single molecules and enables the spatial and temporal understanding of where specific molecules are located at a single cell level.…”