Selective Autonomous Molecular Transport and Collection by Hydrogel‐Embedded Supramolecular Chemical Gradients

Abstract

Selective transport and concentration of molecules to specified regions on asubstrate both enhances the potential to detect such molecules and provides ap ath to spatially localizes uch molecules prior to initiation of subsequent chemical reactions.H ere,w ef irst embed radially symmetric a-, b-, and g-cyclodextrin gradients in ah ydrogel matrix. Driven by host-guest interactions between the cyclodextrins and the target molecule,weobserve these gradients can serve to direct 2D molecular transport. Using xanthene dyes and organophosphates as target molecules,wef ound the transport metrics,e .g., selectivity,r ate,a nd concentration limits,a re strongly dependent on the specific cyclodextrin forming the gradient. In all cases,asthe concentrating power of the gradient increased, the rate of target concentration slowed, whichw e hypothesize is because stronger interactions between the target and the cyclodextrin decrease the rate of target diffusion. The concentration enhancement for the nerve agent simulant 4-methylumbelliferyl phosphate (15.8) is the greatest when the gradient is formed using b-cyclodextrin while directed concentration of cyanomethyl phosphonate,as maller nonaromatic organophosphate,isobserved only for the smaller a-CD.T oprovide anear real-time read-out of the concentration of the analyte,w eu sed an array of IR resonant metallic nanoantennas tuned to as pecific IR absorption band of the analyte to enhance the IR signal generated by the analyte.The targeted transport and concentration of chemical and biological species to specified locations on surfaces has been suggested as an approach to enhance the detection limit of sensors constrained by diffusion-limited analyte collection. [1] Common methods to enhance analyte collection include pumped microfluidic systems [2] and application of electric or magnetic fields (for charged or magnetically-tagged analytes). [3] To enable autonomous molecular transport, approaches utilizing internal driving forces have been explored. Kinesin-based molecular motors have been shown to transport bonded cargos, [4] and surfaces containing chemical gradients have been used to transport DNA, [5] ions, [6] multivalent ligand molecules, [7] dendrimers, [8] poly(ethylene glycol), [9] but with the limitation that the system must be immersed in liquid to enable the transport. Recently,w e reported the use of water-swollen (but not immersed in water) hydrogels films containing built-in charge or hydrophilicity gradients to transport and concentrate molecules. [10] However,t he selectivity of the gradients in those cases were limited, as they only relied on charge or polarity to effect molecular transport.Here,weembedded radially symmetric cyclodextrin (CD) gradients within hydrogel films and showed that these gradients can direct targeted molecules,i ncluding am odel organophosphate,t oc oncentrate within specific regions of the substrate via CD-target supramolecular interactions. Experiments and theory demonstrate that both the transport rate and molecular concentration en...