Molecular recognition is a recurrent issue in chemical sensing because of the importance of selectivity for sensor performances. The progress made in mastering weak interactions has enabled the design of a large pool of synthetic receptors, according to the analytes to be detected, which however has not had a significant impact on sensors technology. This gap has emerged because of the difficulties in transferring the intrinsic molecular recognition properties of a given receptor from solution to interfaces, preserving the functional conformation and orienting the binding fragments toward the analyte-containing phase, in order to reduce nonspecific dispersion interactions. The main reason is the lack of characterization techniques capable of directly probing receptor-ligand association at interfaces.
To fill this gap, we report the first in-situ analytical investigation of molecular recognition at the solid-gas and liquid-gas interfaces, carried out through Sum-Frequency Vibrational Spectroscopy (SFVS), a versatile tool for non-invasive probing of any interface accessible by light, with chemical selectivity, intrinsic surface specificity and sub-monolayer sensitivity. In particular, we investigate the complexation capability of surface-bound organic macrocyclic receptors, namely tetraquinoxaline cavitands (QxCav) and ethylene-bridged tetraquinoxaline cavitands (Et-QxBox),1,2 exposed to aromatic and aliphatic compounds at the air-water3 and solid-air4 interfaces. By probing the molecular fragment involved in the host-guest association, SFVS allows to correlate the molecular architecture of the receptor-decorated surface with the orientation of the complexed cognate analyte inside the binding pocket. The study, which can be readily extended to other receptors, interfacial architectures and analytes, proves that SFVS is able to provide a molecular-level understanding of host-guest complexation at interfaces, close to the actual environment for sensing applications.
1. J.W. Trzciński, R. Pinalli, N. Riboni, A. Pedrini, F. Bianchi, S. Zampolli, I. Elmi, C. Massera, F. Ugozzoli and E. Dalcanale ACS Sensors, DOI: 10.1021/acssensors.7b00110
2. N. Riboni, J.W. Trzcinski, F. Bianchi, C. Massera, R. Pinalli, L. Sidisky, E. Dalcanale and M. Careri, Anal. Chim. Acta, 2016, 905, 79.
3. P. Pagliusi, F. Lagugné-Labarthet, D.K. Shenoy, E. Dalcanale and Y.R. Shen, J. Am. Chem. Soc., 2006, 128, 12610.
4. A. Aprile, F. Ciuchi, R. Pinalli, E. Dalcanale and P. Pagliusi, J. Phys. Chem. Lett., 2016, 7, 3022.