Electrolytic Charge Inversion at Programmable CMOS Sensor Interfaces Krishna Jayant, Mark R. Hartman, Joshua B. Phelps, Philip H. Gordon, Dan Luo, Lois Pollack and Edwin C.Kan 1 Electrical and Computer Engineering, 2 Applied and Engineering Physics, 3 Biological Engineering, Cornell University, Ithaca, NY, USA 14850 ABSTRACT Electrochemical interface layer overcharging is experimentally demonstrated at planar MOS sensor interfaces by controlling the surface charge through nonvolatile charge injection. The electric field across the solid-fluid interface is modulated upon floating-gate program/erase and leads to electrolytic charge reversal, for which an analytical model is derived. This electrofluidic gating effect is further used to repel adsorbed DNA, realizing an electrical surface refreshable biosensor. Quasi-static and impedimetric measurements are presented for validation. KEYWORDS Charge inversion, CMOS, floating gate, double layer MOTIVATION Understanding interactions between charged polymers and surfaces is paramount in fields including the study of DNA/lipid, protein/cell membrane, DNA-protein interactions and electronic microarrays. While traditionally fluorescence and atomic force microscopy (AFM) have been the methods of choice for such interactions, the sensitivity and resolution are limited. Over the last decade CMOS-based sensors have become a powerful tool to monitor charge based interactions [1], pushing label-free detection resolution to the single molecule [2]. In this paper, we will focus on a technique to achieve electrostatic control over molecular adsorption and desorption at the pixel level, which serves the dual purposes of both a surface refreshable biosensor and a test bed for fundamental studies in molecular biophysics. We highlight a revised solid-fluid interface theory that is central to the nature of the proposed mechanism of molecular manipulation. PRINCIPLE OF OPERATION The Debye-Huckel (DH) approximation within the Guoy Chapman (GC) double-layer framework states that the ionic screening effectively lowers the observed charge on molecules in electrolyte as observed from a finite distance. This is the net charge looking into the Gaussian sphere around the molecule and its screening counter ion cloud. However, in strongly correlated liquids (SCL), over-screening can lead to polarity reversal around the molecular Gaussian sphere, which cannot be explained by conventional mean-field theories. This counterintuitive phenomenon is termed as charge inversion (CI) [3-4]. We present here for the first time the experimental evidence of CI on planar MOS sensing surfaces through nonvolatile charge injection using chemoreceptive neuron MOS (CνMOS) sensors (Fig. 1) [1]. Figure 1. Schematic of the CνMOS transistor with capacitively coupled control and sense gates. Nonvolatile charge injection is realized through Fowler- Nordheim (FN) tunneling of either electrons or holes into the electrically floating gate, which is capacitively coupled to the sensing interface and modulates the local ionic population of the double layer by short-range electrostatic forces. This induces CI at the sensing gate due to excessive counter ion polarization which in turn affects the readout current. The electrofluidic gating mechanism is further employed to manipulate surface-bound DNA (Fig 2). Adsorption of DNA first modulates the drain current as it increases the threshold voltage V th owing to the intrinsic negative charge. However, upon floating-gate programming (electron injection) we notice a recovery of the drain current indicating a refreshed sensing surface. We performed impedance spectroscopic measurements to further ascertain the nature of molecular manipulation. Figure 2. (a) DNA adsorption when the device is in the erased state (holes injected), and (b) device in the programmed state (electrons injected) leading to CI and subsequent DNA desorption. A significant improvement over existing impedimetric approaches is the decoupled DC and AC signals applied through the respective control and solution gates. The control gate sets up the quiescent operating point while the solution gate sweeps the frequency of small-signal perturbation to monitor accurately the capacitance at the sensing interface. a. b. W3P.031 978-1-4577-0156-6/11/$26.00 ©2011 IEEE Transducers’11, Beijing, China, June 5-9, 2011 2098