Abstract
An advanced optofluidic system for protein detection based on Raman signal amplification via dewetting and molecular gathering within temporary mesoscale assemblies is presented. The evaporation of a microliter volume of protein solution deposited in a circular microwell precisely follows an outward-receding geometry. Herein the combination of liquid withdrawal with intermolecular interactions induces the formation of self-assembled molecular domains at the solid-liquid interface. Through proper control of the evaporation rate, amplitude of the assemblies and time for spectral collection at the liquid edge are extensively raised, resulting in a local enhancement and refinement of the Raman response, respectively. Further signal amplification is obtained by taking advantage of the intense local electromagnetic fields generated upon adding a plasmonic coating to the microwell. Major advantages of this optofluidic method lie in the obtainment of high-quality, high-sensitivity Raman spectra with detection limit down to sub-micromolar values. Peculiarly, the assembled proteins in the liquid edge region maintain their native-like state without displaying spectral changes usually occurring when dried drop deposits are considered.
Highlights
Optofluidic devices mainly consist of an integration of a microfluidic process with a photonic system acting synergically to maximize activity and performance of chemical and biological analyses[1,2]
The drying process of the fluid suspension is noticed to pass through three main stages before complete drying in accordance with previous findings[24,25]: (I) concentration of solute at the moving contact line, (II) formation of a temporary assembly of solute at the edge of the contact line, and (III) surface adhesion and desiccation onset of the wet assembly
Once the most convenient experimental parameters for promoting molecular assembly at the liquid edge have been established, we evaluated the benefit of adding a Surface-enhanced Raman (SER)-active interface in order to further boosting the Raman signal
Summary
Optofluidic devices mainly consist of an integration of a microfluidic process with a photonic system acting synergically to maximize activity and performance of chemical and biological analyses[1,2]. The interplay of solvent evaporation and solute interaction with the surface can eventually generate molecule deposition in dense macroscopic patterns upon complete drying of the solution, which are frequently referred as “coffee-ring” stains These represent favourable regions for effective detection of analytes in dry conditions, which has long attracted a large interest from the scientific community and exploited in the context of so-called drop-coating deposition Raman methods[10,11,12,13,14]. On getting control over the evaporation-induced molecular flux, the formation of highly dense domains of proteins at liquid edges in their native conformation is favoured Upon inspection of these crowded protein assemblies, an overall gain of ~105 in the Raman signal is produced with respect to that from bulk solution when identical acquisition parameters are used. The proposed system paves the way toward advanced biochemical and biological investigations, as well as shows potential for ultrasensitive detection of biomolecules in their own environment
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