Designing a sensitive and quantifiable nanocolloid assay with dielectrophoretic crossover frequencies

Dielectrophoretic nanocolloid assay is a promising technique for sensitive molecular detection and identification, as target molecule hybridization onto the probe-functionalized nanocolloids can change their surface conductance and consequently their dielectrophoretic crossover frequencies. Thus, instead of relying on surface charge density increase after hybridization, as in many capacitive and field effect transistor impedance sensing techniques, the current assay utilizes the much larger surface conductance (and dielectrophoresis crossover frequency) changes to effect sensitive detection. Herein, we present a Poisson–Boltzmann theory for surfaces with finite-size molecular probes that include the surface probe conformation, their contribution to surface charge with a proper delineation of the slip and Stern planes. The theory shows that the most sensitive nanocolloid molecular sensor corresponds to a minimum in the dielectrophoretic crossover frequency with respect to the bulk concentration of the molecular probes (oligonucleotides in our case) during nanocolloid functionalization. This minimum yields the lowest number of functionalized probes that are also fully stretched because of surface probe-probe interaction. Our theory provides the surface-bulk oligonucleotide concentration isotherm and a folding number for the surface oligonucleotide conformation from the crossover frequency, the zeta potential, and the hydrodynamic radius data.     

Dynamic double layer effects on ac-induced dipoles of dielectric nanocolloids

Normal and tangential surface ionic currents around low-permittivity nanocolloids with surface charges are shown to produce three different conductive mechanisms for ac-induced dipoles, all involving dynamic space charge accumulation at the double layer∕bulk interface with a conductivity jump. However, the distinct capacitor dimensions and diffusive contributions produce three disparate crossover frequencies at which the induced dipole reverses direction relative to the bulk field. A highly conducting collapsed diffuse layer, with bulk ion mobility, renders the particle conductive and produces an ionic strength independent crossover frequency for weak electrolytes. A precipitous drop in crossover frequency occurs at high ionic strengths when charging occurs only at the poles through field focusing around the insulated colloid. A peculiar maximum in crossover frequency exists between these two asymptotes for colloids smaller than a critical size when normal charging of the diffuse layer occurs over the entire surface. The crossover frequency data for latex nanocolloids of various sizes in different electrolytes of wide ranging ionic strengths are collapsed by explicit theoretical predictions without empirical parameters.