Hydrophobic Interaction and Charge Accumulation at the Diamond-Electrolyte Interface

The hydrophobic interaction of surfaces with water is a well-known phenomenon, but experimental evidence of its influence on biosensor devices has been lacking. In this work we investigate diamond field-effect devices, reporting on Hall effect experiments and complementary simulations of the interfacial potential at the hydrogen-terminated diamond/aqueous electrolyte interface. The interfacial capacitance, derived from the gate-dependent Hall carrier concentration, can be modeled only when considering the hydrophobic nature of this surface and its influence on the structure of interfacial water. Our work demonstrates how profoundly the performance of potentiometric biosensor devices can be affected by their surfaces’ hydrophobicity.

Figure 1

(a) Simulated band diagram of the hydrogen-terminated diamond/electrolyte interface. The Fermi level $E_F$ can be shifted below the valence band edge $E_V$ at $-4.45\mathrm{eV}$ with a gate potential $U_G$ applied between the reference electrode ${\overline{\mu }}_{\mathrm{RE}}$ and a contact on the diamond. In the resulting potential well carriers accumulate in a two-dimensional hole gas (2-DHG), leading to band bending at the surface. The hole density is mirrored by a net negative ion density on the electrolyte side. (b) Sheet conductivity versus gate potential acquired from a four-contact resistance measurement using the van der Pauw configuration. See the text for details.

Figure 2

(a) Sheet carrier density versus gate voltage, calculated from in-liquid Hall effect measurements. Above the threshold voltage, the three samples exhibit almost the same slope, which is determined by the interfacial capacitance. (b) Top: Water density profile at a hydrophobic and hydrophilic surface, as from molecular dynamics simulations [15]. Bottom: Ion density profile in the extended Poisson-Boltzmann model for 50 mM NaCl and a hole concentration of $5\times {10}^{12}{\mathrm{cm}}^{-2}$ in the diamond.

Figure 3

(a) ${\mathrm{nextnano}}^3$ simulations showing the potential drop across the diamond/electrolyte interface and the corresponding position of the valence band (VB) with respect to the Fermi level. (Only the region around the interface $\pm 3\mathrm{nm}$ is displayed; arrows indicate the potential drop in the electrolyte.) The PB model (olive) assumes the bulk properties of the electrolyte (${ϵ}_r=78$) up to the interface. The ePB model (red) allows for a depletion of electrolyte in the vicinity of the hydrophobic surface. (b) Comparison between the simulated results [(a)] and the experimentally obtained carrier concentrations [Fig. 2a], for different gate potentials. All data sets are normalized with respect to their threshold voltage. The ePB for the hydrophobic interface, the conventional PB model, and the ePB model for a hydrophilic interface are compared. Results of the ePB model for the hydrophobic case, when a shift of the water density of $\pm 0.5\AA$ is considered, are additionally shown (cyan and magenta).