Abstract
Trace detection based on surface-enhanced Raman scattering (SERS) has attracted considerable attention, and exploiting efficient strategies to stretch the limit of detection and understanding the mechanisms on molecular level are of utmost importance. In this work, we use ionic liquids (ILs) as trace additives in a protein-TiO2 system, allowing us to obtain an exceptionally low limit of detection down to 10–9 M. The enhancement factors (EFs) were determined to 2.30 × 104, 6.17 × 104, and 1.19 × 105, for the three systems: one without ILs, one with ILs in solutions, and one with ILs immobilized on the TiO2 substrate, respectively, corresponding to the molecular forces of 1.65, 1.32, and 1.16 nN quantified by the atomic force microscopy. The dissociation and following hydration of ILs, occurring in the SERS system, weakened the molecular forces but instead improved the electron transfer ability of ILs, which is the major contribution for the observed excellent detection. The weaker diffusion of the hydrated IL ions immobilized on the TiO2 substrate did provide a considerably greater EF value, compared to the ILs in the solution. This work clearly demonstrates the importance of the hydration of ions, causing an improved electron transfer ability of ILs and leading to an exceptional SERS performance in the field of trace detection. Our results should stimulate further development to use ILs in SERS and related applications in bioanalysis, medical diagnosis, and environmental science.
Highlights
Trace detection of proteins plays a major role in a variety of medicinal sectors, and the development of sensitive and specific methods is essential for ultrasensitive bioanalysis and comprehensively understanding the biological processes.[1−4] The development of highly sensitive enhanced Raman spectroscopic techniques in the last two decades has been remarkable
The three different systems studied in this work are shown in Figure 1. 3.1
Molecular force of one single Cyt c molecule interacting with the TiO2 surface, where the values were 1.65, 1.32, and 1.16 nN corresponding to the systems without ionic liquids (ILs), with ILs in phosphate-buffered saline (PBS), and with ILs immobilized on TiO2, respectively
Summary
Trace detection of proteins plays a major role in a variety of medicinal sectors, and the development of sensitive and specific methods is essential for ultrasensitive bioanalysis and comprehensively understanding the biological processes.[1−4] The development of highly sensitive enhanced Raman spectroscopic techniques in the last two decades has been remarkable. Using additives has been proposed as a desirable method to adjust the environment and to regulate the interactions between the adsorbed molecules and substrates to modify their properties Due to their unique physicochemical properties (high ionic conductivity, unique and tunable chemical structures, thermal and chemical stability, etc.), ionic liquids (ILs) have rapidly established themselves in the field of bioanalytical protein chemistry during the last two decades.[30−33] Using these new green solvents to perform protein detection has received much attention, in particular for identifying the low-abundance analytes in biological samples.[34,35] Previous works have mainly focused on using ILs as surface modifiers (i.e., ILs-functionalized or modified substrates) to improve the sensitivity and the limit of detection,[36,37] in which the modification process is usually complex.[38] To the best of our knowledge, the effect of adding ILs into the SERS measurement as additives to adjust the environment has not been investigated. AFM is used as a powerful tool to detect both adhesion and friction forces for obtaining and verifying the interaction strength to clarify the mechanism at the molecular level
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