Biosensor utilizing a liquid crystal/water interface functionalized with...

The interface between the nematic liquid crystal, 4-cyano-4'-pentylbiphenyl (5CB) and water within a transmission electron microscopy (TEM) grid cell coated with the pH-dependent weak cationic amphiphilic block copolymer poly((4-cyanobiphenyl-4'-oxyundecylacrylate)-b-((2-dimethyl amino) ethyl methacrylate)) (LCP-b-PDMAEMA) (which was successfully synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization) was subsequently evaluated for protein and deoxyribonucleic acid (DNA) detection. The LCP-b-PDMAEMA monolayer was fabricated using a Langmuir Blodgett trough, transferred to the 5CB-filled TEM grid, and placed on the octadecyltrichlorosilane-coated glass (TEMPDMAEMA) in such a way that the LCP chains were immersed in the 5CB while the PDMAEMA chains were pointed away from the 5CB surface and immersed in water. Several model proteins such as bovine serum albumin (BSA), hemoglobin (Hb), and chymotrypsinogen (ChTg) were tested at pH values ranging from 2 to 12 to determine the role of the charge state of the protein on protein detection by a weak polyelectrolyte such as PDMAEMA. PDMAEMA contains cationic and neutral states below and above the pKa value, respectively, and is thus able to absorb proteins below its pKa threshold through electrostatic interactions. BSA exhibited a homeotropic to planar (H-P) change in orientation within the TEMPDMAEMA grid cell at concentrations greater than 0.02wt% within the pH range between the isoelectric point (pI) of BSA and the pKa of PDMAEMA, where the charge states of BSA and PDMAEMA were negative and positive, respectively. However, this change in orientation did not occur with other proteins that exhibited a pI higher than the pKa of PDMAEMA due to the electrostatic repulsions resulting from their same cationic charges. This result indicates that the electrostatic interactions between proteins and PDMAEMA are a major contributing factor for protein detection by the H-P transformation within the TEMPDMAEMA grid cell. DNA, a pH-independent strong anionic polyelectrolyte, was also tested with the TEMPDMAEMA grid cell, and it exhibited an H-P transformation at the charged state of PDMAEMA below its pKa threshold at concentrations higher than 0.01wt%. Thus, we demonstrated that the TEMPDMAEMA grid cell effectively facilitated the detection of negatively charged biomaterials (i.e.; protein and DNA) through the H-P transformation using the polarized optical microscope. This simple and inexpensive experimental set-up for non-specific biomaterial detection lays the basic groundwork for developing effective biosensors using polyelectrolytes.