Digvijay Raorane

Evolutionary Screening of Biomimetic Coatings for Selective Detection of Explosives

Susceptibility of chemical sensors to false positive signals remains a common drawback due to insufficient sensor coating selectivity. By mimicking biology, we have demonstrated the use of sequence-specific biopolymers to generate highly selective receptors for trinitrotoluene and 2,4-dinitrotoluene. Using mutational analysis, we show that the identified binding peptides recognize the target substrate through multivalent binding with key side chain amino acid elements. Additionally, our peptide-based receptors embedded in a hydrogel show selective binding to target molecules in the gas phase. These experiments demonstrate the technique of receptor screening in liquid to be translated to selective gas-phase target binding, potentially impacting the design of a new class of sensor coatings.

Polymer-oligopeptide composite coating for selective detection of explosives in water.

The selective detection of a specific target molecule in a complex environment containing potential contaminants presents a significant challenge in chemical sensor development. Utilizing phage display techniques against trinitrotoluene (TNT) and dinitrotoluene (DNT) targets, peptide receptors have previously been identified with selective binding capabilities for these molecules. For practical applications, these receptors must be immobilized onto the surface of sensor platforms at high density while maintaining their ability to bind target molecules. In this paper, a polymeric matrix composed of poly(ethylene-co-glycidyl methacrylate) (PEGM) has been prepared. A high density of receptors was covalently linked through reaction of amino groups present in the receptor with epoxy groups present in the co-polymer. Using X-ray photoelectron spectroscopy (XPS) and gas-chromatography/mass spectroscopy (GC/MS), this attachment strategy is demonstrated to lead to stably bound receptors, which maintain their selective binding ability for TNT. The TNT receptor/PEGM conjugates retained 10-fold higher TNT binding ability in liquid compared to the lone PEGM surface and 3-fold higher TNT binding compared to non-specific receptor conjugates. In contrast, non-target DNT exposure yielded undetectable levels of binding. These results indicate that this polymeric construct is an effective means of facilitating selective target interaction both in an aqueous environment. Finally, real-time detection experiments were performed using a quartz crystal microbalance (QCM) as the sensing platform. Selective detection of TNT vs DNT was demonstrated using QCM crystals coated with PEGM/TNT receptor, highlighting that this receptor coating can be incorporated as a sensing element in a standard detection device for practical applications.

Quantitative and label-free technique for measuring protease activity and inhibition using a microfluidic cantilever array.

We report the use of a SiN x based gold coated microcantilever array to quantitatively measure the activity and inhibition of a model protease immobilized on its surface. Trypsin was covalently bound to the gold surface of the microcantilever using a synthetic spacer, and the remaining exposed silicon nitride surface was passivated with silanated polyethylene glycol. The nanoscale cantilever motions induced by trypsin during substrate turnover were quantitatively measured using an optical laser-deflection technique. These microcantilever deflections directly correlated with the degree of protease turnover of excess synthetic fibronectin substrate ( K M = 0.58 x 10 (-6) M). Inhibition of surface-immobilized trypsin by soybean trypsin inhibitor (SBTI) was also observed using this system.

Nanomechanical assay to investigate the selectivity of binding interactions between volatile benzene derivatives.

Understanding the interactions between aromatic gas molecules and various simple aromatic receptor molecules is important in developing selective receptors for volatile organic compounds (VOCs). Here, five benzene thiols with different functional end groups were used to investigate the weak binding of aromatic vapors such as dinitrotolouene (DNT) and toluene. A multiplexed microcantilever array in conjunction with a very low concentration vapor generation system was developed to study multiple receptor-target interactions simultaneously. Differential nanomechanical responses of such devices provided insight into the influence of various chemical and structural features of such molecules.

Using a Microcantilever Array for Detecting Phase Transitions and Stability of DNA

We report the extension of the microcantilever platform to study the thermal phase transition of biomolecules as they are heated. Microcantilever-based sensors directly translate changes in Gibbs free energy due to macromolecular interactions into mechanical responses. We observe surface stress changes in response to thermal dehybridization of double-stranded DNA oligonucleotides that are attached onto one side of a microcantilever. Once the cantilever is heated, the DNA undergoes a transition as the complementary strand melts, which results in changes in the cantilever deflection. This deflection is due to changes in the electrostatic, ionic, and hydration interaction forces between the remaining immobilized DNA strands. This new technique has allowed us to probe DNA melting dynamics and leads to a better understanding of the stability of DNA complexes on surfaces. (JALA 2006;11:222–6)

Nano-Chemo-Mechanical Sensor Array Platform for High Throughput Selective Coating Material Search

Microcantilever (MC) sensors can detect the presence of chemical vapors at very low concentrations based on the surface stress changes generated by the interactions between probe and target molecules on their surfaces [1-2]. The magnitude of the surface stress change depends on the type of interaction taking place which include hydrogen bonding, electrostatic, van der Waals forces, etc. Pinnaduwage et al [2] demonstrated detection of explosive materials at ultra low concentrations (10-30 ppt) using single MC AFM tip coated with a thiol (-SH) self assembled monolayer (SAM). They were able to get highly sensitive and reproducible signals from their MC sensor while detecting chemicals like PETN, RDX, etc. However, they did not demonstrate the specificity of the coating material to explosive materials. Most types of chemical sensors (metal oxide, conductive polymer, carbon nano tube or belt sensors) are known to respond to interfering chemical agents in a similar manner as the target. Bietsch et al [3] used a set of MC’s (1D array) coated with different types of polymers as chemical sensing layers to try and identify unique deflection signatures for each target chemical. The performance of such sensors is expected to degrade during long term usage as the binding force between the polymer coating and the silicon cantilever structure is weak. Furthermore, polymer coating layers are in general not selective to specific target vapors since they have limited chemical and structural information due to the simple repetition of same chemical structure. To increase the selectivity for a particular target vapor, it is necessary to develop coating materials, which have enough chemical/structural information and long term stability specifically for that target. To expedite the screening process for testing several coating materials in parallel, we need a high throughput sensor array platform.Copyright