(505f) A Shear-Enhanced CNT-DEP Nanosensor Platform for Ultra-Sensitive/Selective Protein Quantification with Tunable Dynamic Range: Overcoming Thermodynamic Limitations

Detection and quantification of low-concentration (<nM) proteins in heterogeneous media are plagued by two distinct obstacles: lack of sensitivity due to high dissociation equilibrium constants K_D of most antibody probes in the range of (µM - nM) and non-specificity due to an abundance of non-targets with similar K_D. We report a new nanoscale protein sensor platform that uses irreversible electrokinetic phenomena to overcome these thermodynamic limitations. The sensitivity is achieved with irreversible rare events, driven by a DEP (dielectrophoresis) driven Ab-Ag-Ab ELISA complex association, which can nevertheless be detected with a sensitive CNT electron tunneling conductance sensor design. The selectivity is achieved by working at a critical shear rate that is between the dissociation shear rate of the target and the non-target, such that a few kT difference in their complexification enthalpy can drastically change their dissociation rate. The CNT reporter plays several important roles in this platform: as an inert but sensitive electron tunneling reporter and as a selectivity enhancer by virtue of its large hydrodynamic viscous drag. We design our current protein ELISA platform based on our earlier CNT sensor work for bacteria sensing¹ and DNA sensing². Tunable dynamic range and ultra-sensitivity are achieved here with a CNT-Ag lock-switch electron tunneling titration design.

Specific components of this new protein sensing platform include: 1. Accelerated DEP association of protein target with an Ab probe functionalized onto an electrode pair with high electric field; 2.Transportation and trapping of the second linking Ab probe functionalized CNTs with long-range DC electrophoresis and short-range AC DEP to the electrodes (lock); 3.Selective removal of assembled CNTs with non-targets by an optimized nanoshear protocol; 4.Conductance quantification of remaining bridging CNTs with target linkers (switch). Steps 1 and 2 involve rapidly driven dynamic events to prevent the captured targets from dissociating from the probe to reach the thermodynamic coverage. Optimized shear at the nm-high hydrodynamic slip length in step 3 irreversibly removes CNTs with non-targets due to the large Stokes drag of their high aspect-ratio cylindrical geometry. With only target-linked CNTs remaining, the digital conductance by electron-tunneling across the Ab-Ag-Ab complex allows us reach a detection limit of 100 molecules (100 aM). Irreversible capture and shearing also allow us to tune the dynamic range up to 100 billion (100 pM or 6 decades) by increasing the CNT number. We will also demonstrate carefully designed selectivity studies against non-targets with similar K_D that report orders of magnitude discriminating factors. The data will be analyzed with a Langmuir association theory to quantify the deviation from thermodynamic equilibrium and how much shear reduces the dissociation barrier. Concentration factor and how DEP and electrophoresis reduces the transport time will also be estimated with collapsed and scaled impedance spectroscopy and conductance data.

1. Zhou, R, Wang, P and Chang, H-C, â??Bacteria capture, concentration and detection by AC dielectrophoresis and self-assembly of dispersed single wall CNTs, Electrophoresis, 27, 1375 (2006).
2. Basuray, S, Senapati, S, Aijian, A, Mahon, A R and Chang, H-C, â??Shear and AC field enhanced CNT impedance assay for rapid, sensitive and mismatch-discriminating DNA hybridization, ACS Nano, 3, 1823 (2009).