Gold–Silver Core–Shell Nanoparticle Crosslinking Mediated by Protease Activity for Colorimetric Enzyme Detection

Abstract

Plasmonic nanoparticles produce a localized surface plasmon resonance (LSPR) under optical excitation. The LSPR of nanoparticles can shift in response to changes in the local dielectric environment and produce a color change. This color change can be observed by the naked eye due to the exceptionally large extinction coefficients (10 8 −10 11 M −1 cm −1 ) of plasmonic nanoparticles. Herein, we investigate the optical shifts (i.e., color change) of three unique gold−silver core−shell nanoparticle structures in response to changes in their dielectric environment upon nanoparticle aggregation. Aggregation is induced by a cysteine-containing peptide that has a sulfhydryl near its N and C termini, which crosslinks nanoparticles. Furthermore, we demonstrate that adding proline spacers between the cysteines impacts the degree of aggregation and, ultimately, the color response. Using this information, we construct a colorimetric enzyme assay, where the signal produced from nanoparticle aggregation is modulated by proteolysis. The degree of aggregation and the resulting optical shift can be correlated with enzyme concentration with high linearity (R 2 = 0.998). Overall, this study explores the optical properties of gold−silver core−shell nanoparticles in a dispersed vs aggregated state and leverages that information to develop an enzyme sensor with a spectral LOD of 0.47 ± 0.09 nM.

Ligation of Gold Nanoparticles with Self-Assembling, Coiled-Coil Peptides

“…To evaluate how matrix effects impacted the fluorescence response, we performed the experiment in PB (10 mM, pH = 7.4), PB and trypsin (1000 nM), bovine serine albumin (BSA), human plasma, Dulbecco’s modified Eagle medium (DMEM) for cell culture, and human urine. With no caspase (Figure f), the fluorescence at 668 nm remained suppressed, except for the trypsin sample because it is a serine protease that cleaves indiscriminately after all Lys residues. , After the addition of caspase (200 ng/mL), the fluorescence at 668 nm turned back on, indicating that our CC FRET-based sensor worked in biologically complex media (Figure g).…”

“…With no caspase (Figure 5f), the fluorescence at 668 nm remained suppressed, except for the trypsin sample because it is a serine protease that cleaves indiscriminately after all Lys residues. 53,54 After the addition of caspase (200 ng/mL), the fluorescence at 668 nm turned back on, indicating that our CC FRET-based sensor worked in biologically complex media (Figure 5g).…”

Optical biosensing systems for a biological living body

“…In addition to interparticle distance, the form of NPs plays a crucial role in maintaining the readability and stability of colorimetric biosensors. Although many forms of NPs including nanopillar, 35 nanostar, 36,37 and core-shell structure, [38][39][40] are used in colorimetric biosensing systems, nanosphere is the most common form because of its isotropy. 41,42 NP accumulation reactions are also applied in point-of-care devices including lateral flow assay (LFA) and paper-based colorimetric biosensing systems.…”

Ultra-stable silver nanoplates: efficient and versatile colorimetric reporters for dipstick assays

“…Indeed, AgNPls are a promising class of silver-based nanomaterials, 28 which, compared to spherical particles, display enhanced optical properties that can be easily tuned via structural changes such as edge sharpness or aspect ratio. [29][30][31] While AgNPls are classically used for SERS applications, 32 they could also be exploited for colorimetric assays as (i) they can offer a multitude of colors 33 and (ii) a lower amount of material could be used per test thanks to their higher extinction coefficient than corresponding spherical particles. However, the use of AgNPls in biosensing applications is even more limited than that of AgNPs, due to their even lower stability.…”