A unified approach based on dielectrophoresis and SERS for protein transport and sensing

The development of single-molecule sensing techniques relies heavily on devices innovation. The key component of these sensing devices is their ability to transport objects in and out of the sensing area. Ultimately, this must ensure delicate and fast spatial manipulation, down to the nanoscale. Dielectrophoresis (DEP) has established itself as a particularly well-suited experimental technique that meets this requirement. However, the current DEP theory does not provide a suitable accuracy to model molecular DEP behaviour, especially for proteins. Nevertheless, experimental protein manipulation by DEP has been efficiently conducted since the 1990s. To support a smooth transition from the mature DEP framework at the microscale towards applications for nanosized molecular particulates, in this presentation, we focus on acquiring the currently missing experimental data on the magnitude of the so-called Clausius-Mossotti (CM) factor, which defines the protein polarizability in a nonuniform electric field and, subsequently, the DEP force. To achieve this, we utilize an innovative dielectrophoretic platform in the form of metallic sawtooth microelectrode arrays with various gap distances between adjacent electrode pairs (Figure 1). The CM factor is determined for three fluorescently labeled proteins (lysozyme, bovine serum albumin (BSA), and lactoferrin) through the measurement of the fluorescence intensity after they have been trapped near electrodes. This way, we can identify the minimum electric field gradient required to overcome dispersive forces (Figure 2). By careful elimination of various sources of experimental errors, we show the significant discrepancy between the obtained CM values and the current DEP model predictions. The results obtained in this work may serve as a quantitative reference to guide further developments in protein DEP theories. We also combine our DEP platform with Ag nanoparticles fabricated by the solid-state dewetting approach (Figure 3) and acquire surface-enhanced Raman scattering (SERS) spectra of BSA (Figure 4) to reveal the potential of applying DEP for upgrading current sensing protocols. A long-term objective is to construct a composite microfluidic device capable of simultaneous particle separation, transport, trapping, and sensing. Such a device may not only push the limits of the forefront micro- and nanosensing techniques but also become extremely useful for daily life applications.