Chemically-Gated and Sustained Molecular Transport through Nanoporous Gold Thin Films in Biofouling Conditions

Abstract

Sustained release and replenishment of the drug depot are essential for the long-term functionality of implantable drug-delivery devices. This study demonstrates the use nanoporous gold (np-Au) thin films for in-plane transport of fluorescein (a small-molecule drug surrogate) over large (mm-scale) distances from a distal reservoir to the site of delivery, thereby establishing a constant flux of molecular release. In the absence of halides, the fluorescein transport is negligible due to a strong non-specific interaction of fluorescein with the pore walls. However, in the presence of physiologically relevant concentration of ions, halides preferentially adsorb onto the gold surface, minimizing the fluorescein–gold interactions and thus enabling in-plane fluorescein transport. In addition, the nanoporous film serves as an intrinsic size-exclusion matrix and allows for sustained release in biofouling conditions (dilute serum). The molecular release is reproducibly controlled by gating it in response to the presence of halides at the reservoir (source) and the release site (sink) without external triggers (e.g., electrical and mechanical).

Bicontinuous Nanoporous Metals with Self-Organized Functionalities

“…20 The good electric conductivity, though often overshadowed by other outstanding properties, has been identified through careful impedance analyses as a crucial factor in the homogenization of the oxidation reaction of BNP Zn anodes 3,13 and the stabilization of BNP Ir during the electrocatalysis of oxygen evolution, 76 partly because in both cases the metals were partly converted to poorly conductive oxides. In the pores, conductivities or diffusivities of common electrolytes and molecules were determined to be generally around 30−50% of the bulk values for BNP Au samples (mostly of φ p ∼ 0.5), 22,60,77 although I cannot further narrow down the range given the differences in the diffusing molecules or ions, the structures, and the measurement methods. All these results support the high conductivities due to the bicontinuity but reveal no clear structural dependence as of yet.…”

“…89 The interface between a BNP metal and an electrolyte, controllable through the electrolyte composition and the applied potential, can thereby influence the transport of ions, 90 enabling selectivity for sensing and controlled molecule release for drug delivery. 22,27 The nanostructures are also versatile substrates for light−matter interactions at the surfaces for a broad range of sensing and photonic applications. 91 At the small length scale, bicontinuity is realized through extremely large negative and positive curvatures, which necessitate many surface defects.…”

“…Though still much larger than the size of ions and small molecules, a pore width of around 10 nm approaches the thickness of the electric double layer of a 1 mM electrolyte estimated by the Gouy–Chapman theory . The interface between a BNP metal and an electrolyte, controllable through the electrolyte composition and the applied potential, can thereby influence the transport of ions, enabling selectivity for sensing and controlled molecule release for drug delivery. , The nanostructures are also versatile substrates for light–matter interactions at the surfaces for a broad range of sensing and photonic applications …”

“…The surface area per weight or volume is inversely proportional to the size of the pores, but that is not the only reason why being nanoporous matters. Uniform throughout a BNP structure (see scanning electron microscopic (SEM) images in Figure 1b and c), the nanometric length scale of both the metal ligaments and the pores leads to high strength, 21 selective transport of large molecules, 22 and exotic defects (Figure 1d) 23 that synergize with the high surface area in catalysis. 24,25 As the title suggests, this perspective focuses on relating functional properties to the formation processes of BNP metals.…”

Anti-Biofouling Effect of Nanoporous Gold Electrode on Nonspecific Signal Reduction for Electrochemical Biosensors

“…Actually, nanoporous gold (np-Au) electrodes exhibited the anti-biofouling effect for albumin [e.g., bovine serum albumin, 10% fetal bovine serum, and human serum albumin (HSA)] and human serum in electrochemical measurements. [21][22][23][24][25][26] This means that the electron transfer that small redox molecules cause at gold surfaces was hindered at planar (bare) gold electrodes but not at np-Au electrodes because more proteins nonspecifically covered the bare gold surface compared with the np-Au surface. It was also reported that the pore size of the np-Au electrode was comparable to that of BSA, showing the anti-biofouling effect.…”