Real-time sensing of proteins, especially in wearable devices, remains a substantial challenge due to the need to convert a binding event into a measurable signal that is compatible with the chosen analytical instrumentation. Impedance spectroscopy enables real-time detection via either measuring electrostatic interactions or electron transfer reactions while simultaneously being amenable to miniaturization for integration into wearable form-factors. To create a more robust methodology for optimizing impedance-based sensors, additional fundamental studies exploring components influencing the design and implementation of these sensors are needed. This investigation addresses a sub-set of these issues by combining optical and electrochemical characterization to validate impedance-based sensor performance as a function of (1) biorecognition element density, (2) self-assembled monolayer chain length, (3) self-assembled monolayer charge density, (4) the electrochemical sensing mechanism and (5) the redox reporter selection. Using a pre-existing lysozyme aptamer and lysozyme analyte combination, we demonstrate a number of design criteria to advance the state-of-the-art in protein sensing. For this model system we demonstrated the following: First, denser self-assembled monolayers yielded substantially improved sensing results. Second, self-assembled monolayer composition, including both thickness and charge density, changed the observed peak position and peak current. Third, single frequency measurements, while less informative, can be optimized to replace multi-frequency measurements and in some cases (such as that with zwitterionic self-assembled monolayers) are preferred. Finally, various redox reporters traditionally not used in impedance sensing should be further explored. Collectively, these results can help limit bottlenecks associated with device development, enabling realization of next-generation impedance-based biosensing with customize sensor design for the specific application.
Embedded contamination (EC) is a significant contributor to front-end-of-line (FEOL) defects detected at Poly Conductor (PC) Bright Field (BF) PLY (Process Limited Yield)inspection after poly patterning. There are two types of EC defects found in PC PLY after poly line formation. A small percentage of them has relatively larger size than ground rule and is believed to be related to particle residue incoming to or fallen during polysilicon deposition. The vast majority of the ECs are small and appear first at post poly deposition Dark Field (DF) PLY inspection as poly bumps. Subsequent PC lithographic pattering and plasma etch modification transform the poly bumps into ECs as detected in PC BF PLY. No appreciable correlation of these small bumps was found to any of the prior level partition BF PLY defects through the defect source analysis (DSA). Larger EC was found to be effectively removed with improved wet clean and the use of cryogenic aerosol clean prior to well anneal process. Small EC was found to be reduced by proper control and minimization of STI height and divot, as well as the processing procedure and environment change.
The detection, collection and/or conversion of biological-based molecules has applications to sensing, wearables, and biofuels. Flexible, wearable electronics for real-time detection of, for example, Neuropeptide Y in human sweat [1], or flagella protein in biofuels, enables deviation from bulky laboratory benchtop analyses such as ELISA. Challenges include required limits of detection, as biological materials can be present in small amounts (nM to pM concentrations) in complex high ionic strength environments[2], the need for a highly selective sensor interface that avoids non-specific binding, and a quick response to account for degradation of analytes. This work will discuss the implementation of a mixed self-assembled monolayer (SAM) interface with highly selective bio-recognition elements (BREs) on gold disk electrodes, focusing on target application for aqueous sensing in biofuels, with room for expansion to sensing in other biological fluids. The SAM composition was systematically varied to assess functionality towards preventing fouling/non-specific binding and increase the selectivity of the interface for the analyte of interest. Briefly, mixed monolayer combinations included pseudo-zwitter surfaces to prevent fouling, blocking groups similarly employed in the literature[3] to help rectify BREs, and dithiobis(succinimidyl propionate) (DSP) cross-linking chemistry[4] to attach various BREs to the gold electrode. BREs include concanavilin A, streptavidin, nanobodies, and synthetic lectins. Streptavidin and it’s interaction with biotin was chosen as a control, due to their known strong interaction. Electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV) were used to estimate changes in the resistance to charge transfer to redox mediators either immobilized or in solution. EIS is one of the most common modern techniques used in biosensing, but as a time-intensive measurement it can be undesirable for accurate sensing in analyte-degrading environments. Therefore, SWV was employed as a substitute which reduces times of measurements by an order of magnitude, and is shown to provide the same information as EIS. To corroborate these electrochemical effects with a definite surface coverage/binding event, ellipsometric experiments of functionalized electrodes are underway. Our SAM interface produces nM level detection for yeast through the employment of gold immobilized charge transfer mediators and concanavalin A with selective binding to the yeast cell wall. Specifically, a decrease in charge transfer resistance (or increase in peak current as measured in SWV) occurs upon binding of yeast cells to concanavalin A. The streptavidin-biotin interaction also manipulated the charge transfer resistance similarly, confirming our method effectiveness. Nanobodies (binding fragment of antibodies) were also shown to have some success in detecting flagella protein with redox mediators in solution. These proof-of-concept results demonstrate feasibility and indicate forward paths towards highly selective sensor electrodes with simultaneous anti-fouling properties that prevents non-specific binding. Ongoing efforts include modifying electronic device interfaces with SAMs for combined signaling and amplification, such as modification of gate electrodes in printable electrochemical transistors. References: [1] G. Cizza et al., “Elevated Neuroimmune Biomarkers in Sweat Patches and Plasma of Premenopausal Women with Major Depressive Disorder in Remission: The POWER Study,” Biol. Psychiatry 64, 907–911 (2008); doi: 10.1016/j.biopsych.2008.05.035 [2] Z. Sonner et al., “The microfluidics of the eccrine sweat gland, including biomarker partitioning, transport, and biosensing implications,” Biomicrofluidics 9, 031301(2015); doi: 10.1063/1.4921039 [3] R. Levicky, T. M. Herne, M. J. Tarlov, and S. K. Satija, “Using Self-Assembly To Control the Structure of DNA Monolayers on Gold: A Neutron Reflectivity Study,” J. Am. Chem. Soc. 120, 9787–9792 (1998); doi: 10.1021/ja981897r [4] A. J. Lomant and G. Fairbanks, “Chemical probes of extended biological structures: Synthesis and properties of the cleavable protein cross-linking reagent [35S] dithiobis(succinimidyl propionate),” J. Mol. Biol. 104, 243–261, (1976); doi: 10.1016/0022-2836(76)90011-5 Figure 1
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