Stretchable Sensors for Nanomolar Glucose Detection

Abstract

surface area. [12,13] To overcome this issue, nanostructured electrodes with small geometric footprints have been fabricated to achieve higher surface area than their planar counterparts. [14][15][16][17] Wearable sensors in particular should be biocompatible and withstand physiological strains due to motion and deformation of human skin, which is ≈30%. [18,19] Stretchable gold electrodes have been evaluated as glucose detection platforms. Chan et al. demonstrated stability at strains up to 230% with their solution-processed wrinkled gold glucose sensor, with a limit of detection (LOD) of 1 × 10 −3 m in artificial sweat. [20] Zhai et al. reported gold nanowire-based electrodes functionalized with glucose oxidase that were stretched to 20% strain to detect glucose concentrations of 1 × 10 −5 m in a solution of NaOH, while Zhao et al. demonstrated a gold-fiber based sensor functionalized with glucose oxidase capable of detecting 6 × 10 −6 m glucose in phosphate-buffered solution (PBS) under strains up to 200%. [21,22] It is critical to have low detection limits and high sensitivity due to lower concentration of glucose in sweat in comparison with other biological fluids. In this work, we introduce a highly stretchable electrode with the lowest limit of detection to the best of our knowledge for flexible enzyme-free glucose sensing at physiological pH.We introduce stretchable wrinkled electrodes for electrochemical sensing by depositing thin gold metal thin film on polyolefin (PO), which shrinks to 5% of its original area. During the shrinking process, the stiffness mismatch between the polymer and the gold thin film leads to buckling of the gold and results in hierarchical, wrinkled structures. The shrinking factor (the ratio of the average unshrunk electrode's geometric area to that of the processed electrode) of the prestressed thermoplastic PO is 21.8. The wrinkled thin film is then transferred to an elastomer. The transfer of the electrode from shape-memory polymer to elastomer results in additional shrinking due to lift-off process, with a total shrinking factor of 33.4 (Figure 1a). The transferred electrodes retain the wrinkled structures (Figure 1b-d) and upon application of strain, the wrinkles stretch and separate from each other as cracks are sustained in the gold thin film (Figure 1e-g).Electrochemical properties of the wrinkled electrodes were characterized by measuring the electrochemical active surface area (EASA) in different solutions. The wrinkled structures contribute to high surface area, which directly correlates Biosensors that detect analytes in sweat face the challenge of maintaining sensitivity upon miniaturization. Various materials and processes have been developed to create nanostructured electrodes with high surface areas to mitigate this issue. The need remains, however, for biocompatible materials that can be scalably integrated into wearable devices. This paper details a gold thin-film electrode fabricated using a thermoplastic shape memory polymer to create hierarchical wrinkled stru...

Detection of the SARS-CoV-2 spike protein in saliva with Shrinky-Dink© electrodes

“…We have previously demonstrated signal enhancement in the form of greater dynamic range of detection on electrodes made using the '80's children's toy, Shrinky-Dink ©, which saturates the color of a drawing made on a thermoplastic polystyrene sheet that shrinks when heated ( Figure 1). 31,32 Our group published on Shrinky-Dink Microfluidics in this journal in 2008 and it has since become a workhorse in laboratories around the world. 33,34 In this study, we demonstrate using Shrinky-Dink wrinkled (SDW) electrodes for sensitive detection of the S1 subunit of the spike protein of SARS-CoV-2 with the purpose of providing a low-cost screening and diagnostic device for the COVID-19 infection.…”

“…The wrinkled gold surface has been extensively investigated in previous publications from our group and others. 31,32,[35][36][37][38][39] To determine the functionality and affinity of the aptamers in the E-AB format, all initial experiments were first performed on commercial disc (CD) Au electrodes (CH Instruments, 2 mm diameter) in a standard 3-electrode electrochemical cell ( Figure 2A). The commercial gold electrodes were functionalized with the aptamers and backfilled with blocking MCH monolayers.…”

“…The wrinkles provide a diffusion-based challenge to surface coverage, as previously described. 31 With longer incubation time, however, this limitation was overcome. The outcome was a greater number of moles of aptamers tethered to the surface of SDW electrodes relative to the same geometric area of CD electrodes.…”

“…The aptamer incubation time was optimized for SDW electrodes; from previous investigations, it is clear that the wrinkled morphology presents diffusion limitations for molecular access to the surface. 31 Thus, the incubation time of aptamers was increased to ensure adequate surface coverage by the aptamer probes. During this time, the aptamers formed thiol bonds with the gold surface.…”

Progress of shrink polymer micro- and nanomanufacturing

“…Recently, heat-shrinkable shape memory polymers (SMPs) have captured the attention of researchers in the micro/nanofabrication field 12 , 17 – 20 due to the improved fabrication resolution and rapid and controllable fabrication processes 12 . Heat-shrinkable SMPs, also referred to as heat-shrink polymers or heat-recoverable polymers, were first utilized in the packaging industry 21 .…”