Hydrated Ionic Liquids Boost the Trace Detection Capacity of Proteins on TiO₂ Support

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-TiO 2 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 × 10 4 , 6.17 × 10 4 , and 1.19 × 10 5 , for the three systems: one without ILs, one with ILs in solutions, and one with ILs immobilized on the TiO 2 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 TiO 2 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.

Selective Separation of Highly Similar Proteins on Ionic Liquid-Loaded Mesoporous TiO2

“…For instance, the molecular force of lysozyme with a TiO 2 surface is about 0.86 nN at pH = 7.2, and that for Cyt c with TiO 2 is 1.32 nN, which is in accord with the findings in this work. Meanwhile, the molecular force of Cyt c on the IL-loaded TiO 2 surface is weaker than that on TiO 2 without ILs, which is consistent with the results obtained in this work.…”

“…Even though they were loaded on the TiO 2 surface, the hydrated cations and anions did change the microenvironment in the solution. Meanwhile, these hydrated ions could form a strong hydration layer on the IL-loaded TiO 2 surface, leading to a decrease in the molecular interaction forces and the total adsorption capacity of proteins …”

“…Meanwhile, these hydrated ions could form a strong hydration layer on the IL-loaded TiO 2 surface, leading to a decrease in the molecular interaction forces and the total adsorption capacity of proteins. 38 The formation of the hydration layer takes place on all the IL-TiO 2 surfaces, while the hydration strength is different. Meanwhile, after hydration, a weak diffusion of the hydrated ions for the IL loaded on TiO 2 is unavoidable.…”

Molecular Mechanistic Insights into the Ionic-Strength-Controlled Interfacial Behavior of Proteins on a TiO2Surface

“…During the last step to attach the Cyt c molecules, the tips were immersed in the Cyt c solutions at each ionic strength, then washed with the corresponding ionic strength solutions, and lastly dried with N 2 . Before the adhesion force measurements, the tip’s normal spring constant should be calibrated with the same procedure as that in our previous work ,, in order to transform the measured-load signals from volts (V) into force signals (N). Approximately 100 force–distance curves were measured to obtain the average values of the adhesion force by recording the maximal adhesion force upon retraction.…”

“…), finding that these molecular forces depend both on the properties of the protein and the solid surface and the pH conditions but not the surface roughness. Importantly, these determined molecular forces can be further used as a guideline to design the macroscopic experiments, including protein separation measured by highperformance liquid chromatography, 33 surface-enhanced Raman scattering (SERS)-based protein biodetection, 35 and electron transfer ability of the enzyme tested using an electrochemical biosensor. 34 We found that the magnitude of the ionic strength, induced by the buffer ions, significantly alters the interfacial process performance, while how the ionic strength affects the molecular interaction at the microscale and then influences the performance at the macroscale has not yet been clarified.…”