Nanoliter Sensing for Infrared Bioanalytics

Abstract

Nondestructive label-free bioanalytics of microliter to nanoliter sample volumes with low analyte concentrations requires novel analytic approaches. For this purpose, we present an optofluidic platform that combines surface-enhanced in situ infrared spectroscopy with microfluidics for sensing of surface-immobilized ultrathin biomolecular films in liquid analytes. Submonolayer sensitivity down to surface densities of few ng/cm is demonstrated for the adsorption of the thiolate tripeptide glutathione and for the recognition of streptavidin on a biotinylated enhancement substrate. Nonfunctionalized and functionalized metal island films on planar oxidized silicon substrates are used for signal enhancement with quantifiable enhancement properties. A single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is used with ordinary planar, IR-transparent windows. The geometry circumvents the strong IR absorption of common polymer materials and of aqueous environments in the IR fingerprint region. This practice enables straightforward quantitative analyses of, e.g., adsorption kinetics as well as chemical and structural properties in dependence of external stimuli.

Large-Area Interfaces for Single-Molecule Label-free Bioelectronic Detection

“…(C) Time evolution of in situ IR spectra showing the streptavidin–biotin binding after SA injection. Adapted with permission from ref . Copyright 2018 American Chemical Society.…”

“…An optofluidic SEIRAS platform enabling submonolayer identification of a protein prototype system comprising streptavidin on a biotinylated substrate in nanoliter volumes has been recently reported. 230 Studying the vibrational spectrum in situ and in real time allows assessing the kinetics of adsorption processes and receptor−ligand binding. In Figure 30A the crosssectional scheme of the microfluidic flow cell shows the islandlike structure of a gold film on a silicon substrate.…”

Microfluidic nanodevices for drug sensing and screening applications

“…(e) Paper-based microfluidic device with wick-like structure for colorimetric screening of anticancer drugs, reproduced with permission from ( Fu et al, 2021 ), copyright 2021 Wiley-VCH. (f) Microfluidic chip integrated with in situ IR spectroscopy for highly sensitive screening of single molecules, reproduced with permission from ( Kratz et al, 2018 ), copyright 2018 American Chemical Society. …”

“…To enhance the sensitivity, microfluidic substrates are often functionalized with metal island films for signal enhancement, which is known as surface-enhanced IR spectroscopy (SEIR). For instance, in 2018, Kratz et al proposed an optofluidic platform by integrating SEIR with microfluidics, which offers potential applications in drug screening by detecting the protein responses to particular drugs ( Kratz et al, 2018 ). Along with the IR-transparent windows, a single-reflection geometry at an incidence angle below the attenuated-total-reflection (ATR) regime is employed, which reduces the strong IR absorption of common polymer materials and aqueous environments in the IR fingerprint region, allowing this configuration to be used for high-throughput drug detection ( Fig.…”

Infrared and Raman spectroscopic analysis of functionalized graphene

“…(a) Difference IR microscopic spectrum: GSH on graphene on gold island film adapted from Kratz [13]; (b) molecular adsorption isotherm during GSH monolayer formation adapted with permission from Kratz et al [29]. Copyright {2018} American Chemical Society; (c) Raman spectra of the Si surface (gray), of the Au on Si sample (black), after transfer of a graphene sheet to the Au/Si substrate (red), and after the functionalization of G/Au/Si with MP (blue); (d) sketch of adsorption of GSH to the MP/graphene/Au interface (corresponding data in a) or directly to the Au island interface (corresponding data in (b)).…”