Designing Silver Nanoparticles for Detecting Levodopa (3,4-Dihydroxyphenylalanine, L-Dopa) Using Surface-Enhanced Raman Scattering (SERS)

Abstract

Detection of the drug Levodopa (3,4-dihydroxyphenylalanine, L-Dopa) is essential for the medical treatment of several neural disorders, including Parkinson's disease. In this paper, we employed surface-enhanced Raman scattering (SERS) with three shapes of silver nanoparticles (nanostars, AgNS; nanospheres, AgNP; and nanoplates, AgNPL) to detect L-Dopa in the nanoparticle dispersions. The sensitivity of the L-Dopa SERS signal depended on both nanoparticle shape and L-Dopa concentration. The adsorption mechanisms of L-Dopa on the nanoparticles inferred from a detailed analysis of the Raman spectra allowed us to determine the chemical groups involved. For instance, at concentrations below/equivalent to the limit found in human plasma (between 10 −7 -10 −8 mol/L), L-Dopa adsorbs on AgNP through its ring, while at 10 −5 -10 −6 mol/L adsorption is driven by the amino group. At even higher concentrations, above 10 −4 mol/L, L-Dopa polymerization predominates. Therefore, our results show that adsorption depends on both the type of Ag nanoparticles (shape and chemical groups surrounding the Ag surface) and the L-Dopa concentration. The overall strategy based on SERS is a step forward to the design of nanostructures to detect analytes of clinical interest with high specificity and at varied concentration ranges.Sensors 2020, 20, 15 2 of 18 atrazine (ATZ) on Ag nanospheres (AgNP) made it possible to detect picomolar concentrations of ATZ using SERS, whose intensity was higher in solution than in a cast film [7]. This also means that the orientation of ATZ molecules on AgNP is crucial for this ultrasensitive analysis. One may infer that signal enhancement for analytes can be obtained by tailoring the size and shape of the nanoparticles [12][13][14], in addition to the conformation of the analyte adsorbed as in the detection of B-complex vitamins in pharmaceutical samples [15].Such control in nanoparticle shape is afforded through various synthetic routes [16][17][18], then allowing the Localized Surface Plasmon Resonances (LSPR) to be tuned according to the region of best response (excitation) of the analyte [14,18,19]. For example, the near electromagnetic field is higher in nanoprisms (AgNPR), nanostars (AgNS) and nanorods (AgNR) than on spherical nanoparticles (AgNP) [18,19], and the LSPR may be wider to allow excitation down to the near infrared (IR) [18][19][20]. Indeed, Rycenga et al. [21] reported higher SERS activity for three molecules adsorbed on Ag nanocubes (AgNC) than on AgNP due to the wider LSPR for AgNC. Izquierdo-Lorenzo et al. [18] found that AgNPR were more efficient than AgNP for detecting aminoglutethimide used in sport doping owing to field enhancement in the corners of AgNPR, forming interstitial junctions, which are called "hot spots".The adsorption (and orientation, as a consequence) of an analyte can in principle be controlled by: (a) the affinity between analyte and metal; (b) charge of the analyte, which can be modulated via pH; (c) size and shape of the nanoparticles, and (d) metallic surface, wh...

Impedimetric and Plasmonic Sensing of Collagen I Using a Half-Antibody-Supported, Au-Modified, Self-Assembled Monolayer System

“…These surface forms have high electrical conductivity and compatibility towards protein receptor molecules [51][52][53][54]. In addition, the preformed surface with its noble metal nanoparticles [55] is a capable environment for maintaining the physiological activity of immobilized proteins. Metal nanoparticle monolayers are suitable for the immobilization of whole molecules of antibodies via electrostatic interactions [52,53], as well as for covalent immobilization of the Fab part of antibodies via Au-S covalent bonds [51,54].…”

“…Metal nanoparticle monolayers are suitable for the immobilization of whole molecules of antibodies via electrostatic interactions [52,53], as well as for covalent immobilization of the Fab part of antibodies via Au-S covalent bonds [51,54]. The main benefit of using the F ab parts of antibodies is the immobilization of the stable sensing elements, ensuring their appropriate orientation [52,55,56].…”

SERS-Based Aptasensor for Rapid Quantitative Detection of SARS-CoV-2

“…There are many different types of SERS substrates [33][34][35][36] and schemes for creating aptasensors based on them [20,[37][38][39][40]. However, to create a quantitative method for the selective detection of SARS-CoV-2 from a group of other respiratory viral infections, this method should be the cheapest, simplest, and most reproducible.…”

The Plasmonic Effect of Gold Nanorods on Charged Molecules: SERRS and SEF effects

“…The latter can change the surface charge, influencing the chemisorption or physisorption of the target molecules onto the nanoparticle surface. In general, metallic nanoparticles show negative charge surfaces, which can difficult the adsorption of anionic molecules 28 , but can favor the adsorption of cationic ones through electrostatic interactions 29 . The gold nanorods (AuNRs) coated with a CTAB bilayer have received an increasing interest in the SERS application due to the presence of transverse and longitudinal plasmon resonance and the positive surface charge (provide by CTAB bilayer) 30 .…”