Molecularly imprinted polypyrrole films for smart voltage-gated molecular uptake and release

Abstract

The controlled uptake and release of biochemical molecules at material interfaces is critical for applications in drug delivery, biosensing, and neurostimulation. While molecularly imprinted polymers (MIPs) offer selective molecular recognition, real-time visualization of electrically controlled molecular trafficking remains underexplored. This study investigates a voltage-gated molecular interface using molecularly imprinted polypyrrole (MIP-PPy) films, designed to enable reversible uptake and release of target molecules with direct optical monitoring. Fluorescein sodium salt, a model for anionic neurotransmitters, was incorporated during electrochemical polymerization to create specific recognition sites within the polymer matrix. We employed real-time fluorescence microscopy to directly track molecule transport dynamics in response to applied voltages, utilizing a custom 3D-printed flow cell for simultaneous electrical control and in situ imaging. The MIP-PPy films demonstrated self-regenerating uptake behavior from the surrounding solution, overcoming the limitations of unidirectional release in conventional microfluidic systems. This bidirectional control introduces new opportunities for reservoir-free, adaptive chemical modulation. Biocompatibility assessments using SH-SY5Y neuroblastoma and Huh7 hepatoma cells demonstrate high levels of viability and adhesion, confirming the films’ suitability for biological integration. Overall, this study provides a visual and quantitative platform for investigating electrically induced molecular transport, which will assist in the development of closed-loop drug delivery and neurostimulation systems inspired by synaptic signaling.

1. Introduction

The controlled uptake and release of biochemical molecules at material interfaces are fundamental to advancing drug delivery systems [1], biosensors [2], and neurostimulation technologies [3,4]. Although significant progress has been made with stimuli-responsive platforms, including liposomes [5], nanoparticles [6,7], and hydrogels [8–10], many conventional approaches rely on passive release or finite-capacity reservoirs, limiting adaptability and long-term utility in dynamic biological environments. Molecularly imprinted polymers (MIPs) provide a promising alternative by enabling selective molecular recognition through template-specific cavities [11,12]. Conductive MIPs, such as molecularly imprinted polypyrrole (MIP-PPy), offer the added benefit of electrochemical responsiveness, allowing external control over molecular uptake and release [13–15]. These features have led to extensive exploration of MIP-PPy in sensing [16,17] and separation applications [18–20] due to its chemical stability, molecular specificity, and integration potential. However, most prior studies have inferred molecular transport behavior using indirect techniques such as quartz crystal microbalance (QCM), cyclic voltammetry (CV), or electrochemical impedance spectroscopy (EIS) [11,21–24]. Direct, real-time visualization of electrically controlled molecule dynamics, particularly under biologically relevant conditions, remains limited.

In this study, we investigate a voltage-gated MIP-PPy interface using fluorescein sodium salt as a model anionic compound representative of neurotransmitters. By incorporating fluorescein during electrochemical polymerization, we created imprinted films on platinum electrodes with defined molecular recognition sites. A custom-designed 3D-printed electrochemical flow cell was employed to enable precise voltage application and in situ fluorescence imaging. This setup allowed us to directly observe, in real time, the reversible release and reuptake of template molecules in response to electrical stimulation.

Our results confirm that MIP-PPy films enable selective, repeatable, and bidirectional transport of target molecules. Notably, the ability to reuptake molecules from the surrounding environment allows self-sustaining operation without reliance on preloaded chemical reservoirs. To evaluate the biocompatibility of the platform with long-term bioelectronic devices, SH-SY5Y neuroblastoma and Huh7 hepatoma cells were employed in biocompatibility analyses. High cell adhesion and viability on the PPy-coated surfaces prove that they are viable for potential integration with living tissue. In general, this work provides a visual and quantitative platform for the study of electrically triggered molecular exchange. It lays the groundwork for developing reservoir-free, programmable chemical interfaces for applications in closed-loop drug delivery, adaptive therapeutics, and synapse-inspired neurostimulation systems such as artificial retinal implants [25–29].

2. Materials and Methods

2.1. Electrochemical Polymerization and Structural Design of MIP-PPy Films

Figure 1 illustrates the simplified structure of the MIP-PPy film, designed to enable voltage-controlled uptake and release of a selectively imprinted molecule on a platinum (Pt) electrode. The structure features a fluorescein sodium salt-imprinted PPy film, electropolymerized onto a 0.5 mm diameter circular Pt electrode. The electrochemical polymerization process (Figure 1(a)) occurs in an aqueous solution containing pyrrole monomers, fluorescein sodium salt (used as the template), and potassium chloride (KCl) as the supporting electrolyte. During polymerization, pyrrole monomers are oxidized at the electrode surface, resulting in the formation of a positively charged PPy backbone. To maintain charge neutrality, negatively charged species—including both the dopant fluorescein and background chloride ions (Cl⁻) from KCl—are incorporated into the polymer matrix. While chloride ions do not participate directly in molecular imprinting, they contribute to charge balance and conductivity within the polymer. Fluorescein, with its aromatic rings, interacts non-covalently with the conjugated pyrrole chains of the PPy backbone through π–π stacking and hydrophobic interactions, stabilizing the template within the growing polymer matrix [30]. These interactions lead to the physical entrapment of fluorescein within the polymer, resulting in a PPy-Fluorescein composite film. After polymerization, the fluorescein molecules can be removed by applying alternating redox potentials, which leaves behind well-defined cavities in the polymer structure that are complementary in shape and chemical functionality to the template, enabling selective rebinding. The uptake and release of fluorescein can be reversibly controlled by applying positive and negative voltages, modulating the electrostatic interactions between the polymer and the target molecule (Figure 1(b)). This study provides the first direct visual confirmation of electrically gated molecular release and uptake in an MIP-based system, offering new insights into the design of programmable chemical interfaces for applications in sensing, drug delivery, and bioelectronics.

Figure 1.

Schematic of the MIP-PPy structure showing (a) electrochemical polymerization and (b) its voltage-controlled template release and uptake.

2.2. Chemicals, Reagents, and Experimental Apparatus

Pyrrole (C4H5N) was purchased from TCI, USA, and stored at freezing temperature. Fluorescein sodium salt (C20H10Na2O5) was obtained from VWR, USA, while pyranine (C16H7Na3O10S3) dye was purchased from Ambeed, USA. KCl was sourced from Ward’s Science, USA. All aqueous solutions were prepared using deionized (DI) water (Reagent Grade, Electron Microscopy Sciences, Inc., USA). Acetone, methanol, and isopropanol were used for cleaning electrodes and glassware. Electrochemical polymerization of pyrrole was performed using an eDAQ EA 163 potentiostat, controlled via eChem and eChart software. The electrolyte solutions were prepared using an HJ-2A constant-temperature magnetic stirrer (VEVOR, USA). A three-electrode electrochemical cell was employed for the fabrication of NIP and MIP structures, consisting of a 0.5 mm diameter circular Pt working electrode, a 5 mm × 5 mm square Pt counter electrode, and an Ag/AgCl reference electrode. Prior to each polymerization, all solutions were purged with nitrogen gas for 10 minutes to remove dissolved oxygen. The working electrode was sequentially cleaned with acetone, methanol, isopropanol, and distilled water, followed by air drying at room temperature. After electrochemical polymerization, both MIP and non-imprinted polymer (NIP) structures were imaged using a Nikon Eclipse L200 optical microscope. Film thicknesses of the MIP and NIP layers were measured using the same optical microscope and a KEYENCE LC-2420 laser displacement meter. Molecular release and uptake studies were conducted using a Nikon Eclipse TE2000-S inverted fluorescence microscope, with flow control provided by an ISMATEC syringe pump.

2.3. Fabrication and Characterization of MIP-PPy and NIP-PPy Films

To prepare the electrolyte solutions for MIP-PPy and NIP-PPy fabrication, a 0.1 M aqueous KCl solution was prepared by dissolving 0.7456 g of KCl in 100 mL of DI water, followed by stirring for 60 minutes. A 50 mM stock solution of fluorescein sodium salt was prepared by dissolving 0.9406 g of fluorescein in 50 mL of the KCl solution. All solutions were prepared in a nitrogen-purged environment to prevent oxidative degradation of pyrrole monomers. For the MIP-PPy solution, fluorescein was added to the KCl solution at a final concentration of 2 mM, and pyrrole monomer was added at 0.2 M. The NIP-PPy solution was prepared similarly, but without the fluorescein template. Electrochemical polymerization was performed in a standard three-electrode electrochemical cell, with a potentiostat controlling the applied potential between a 0.5 mm diameter Pt working electrode, a square Pt counter electrode, and an Ag/AgCl reference electrode, all immersed in the electrolyte solution. The electrodes were cleaned by sequential rinsing with acetone, methanol, and isopropanol, followed by drying with nitrogen gas. A constant potential of 0.8 V was applied for 10 minutes to initiate and sustain electrochemical polymerization, with continuous monitoring of the deposition current to ensure stable film formation. Figure 2(a) illustrates the setup for the electrochemical polymerization process. Figures 2(b) and 2(c) show the cylindrical Pt electrodes before and after electrochemical polymerization. During polymerization, initial sharp increases in current density were observed due to the rapid oxidation of pyrrole monomers. As polymerization proceeded, the current density gradually increased, indicating steady film formation. Figures 2(d) and 2(e) show the current versus time profiles for NIP-PPy and MIP-PPy, with NIP-PPy exhibiting a higher oxidation rate and current due to the absence of the fluorescein template. In contrast, the MIP-PPy solution, which included the fluorescein template, slightly hindered monomer diffusion, leading to a lower current. Figure 2(f) presents the cyclic voltammetry (CV) profiles for bare Pt, NIP-PPy, and MIP-PPy films in 0.1 M KCl, with and without 2 mM fluorescein, at a scan rate of 70 mV/s. The bare Pt electrode exhibited negligible faradaic activity. In the fluorescein-free electrolyte, both PPy films showed distinct redox peaks, characteristic of their electrochemical behavior. The NIP-PPy film displayed higher peak currents, indicative of the uniform chloride-doped PPy matrix, which facilitates efficient ion exchange and charge transport. In contrast, the MIP-PPy film exhibited slightly lower peak currents due to the incorporation of the fluorescein template, which disrupted polymer uniformity. In the electrolyte with fluorescein, NIP-PPy showed a minor current change from non-specific adsorption. However, MIP-PPy exhibited more pronounced peak changes, which are consistent with specific dye binding within imprinted cavities and support its role in selective, voltage-controlled molecular uptake and release. However, the MIP-PPy film retained its electrochemical functionality, crucial for voltage-controlled molecular uptake and release. After electrochemical polymerization, MIP-PPy and NIP-PPy films were stored in DI water for 60 minutes. The PPy films were carefully removed from the electrodes using Scotch tape, and their thickness was measured. The average thickness values were 59.25 ± 8.13 μm for NIP-PPy and 51 ± 7.78 μm for MIP-PPy, based on four measurements for each film type to ensure consistency. Because it influences the density of imprinted sites and the effectiveness of analyte diffusion, controlling film thickness is essential for reliable device operation. While excessively thick films may impede mass transport, reduce response times, and complicate template removal, thereby jeopardizing stability, too thin films may lack sufficient recognition sites or exhibit inadequate cavity formation [31,32]. Deposition parameters were adjusted to produce uniform film morphology and reproducible thicknesses.

Figure 2.

(a) Diagram of the three-electrode setup used for PPy film deposition on Pt electrodes, Optical images of Pt electrodes (b) before and (c) after NIP/MIP deposition, Current-time plots during (d) NIP-PPy and (e) MIP-PPy electrochemical polymerization and (f) CV profiles of bare Pt, NIP-PPy, and MIP-PPy electrodes in 0.1 M KCl with and without 2 mM fluorescein.

3. Results and Discussion

3.1. Fluorescence Calibration and Imaging Setup

To investigate the release and uptake behavior of MIP-PPy film and NIP-PPy film, we utilized the experimental setup shown in Figure 3(a). The left image is a schematic representation, while the right captures the custom-built, 3D-printed electrochemical flow cell mounted on an inverted fluorescence microscope. The configuration includes a Pt working electrode coated with MIP-PPy film or NIP-PPy film, a Pt wire counter electrode, and a reference electrode, all immersed in a 0.1M KCl electrolyte solution. The flow cell accommodates appropriate inflow and outflow using an ISMATEC syringe pump, with a flow rate set at 10 μl/min to minimize excess fluorescein accumulation in the observation zone during the real-time monitoring of actively released dye molecules on the MIP-PPy surface. A UV light source is directed through a dichroic mirror and focused onto the sample via an objective lens, exciting fluorescein molecules. The emitted fluorescence, centered around 520 nm, is captured by a camera located beneath the setup. This optical arrangement facilitates real-time visualization and quantification of fluorescein release and uptake from the imprinted sites of the polymer in response to applied electrical potentials.

Figure 3.

(a) Custom 3D-printed flow cell on a fluorescence microscope. (b) Fluorescence calibration curve. (c–d) Images after voltage-triggered release and uptake.

Figure 3(b) presents the calibration curve of fluorescence intensity versus fluorescein concentration, which was used to quantify dye release and uptake during subsequent measurements. Figure 3(c) shows a fluorescence microscope image of the MIP-PPy film deposited on a Pt electrode after applying a reduction voltage of −0.4 V for 1 second, resulting in fluorescein dye release from the polymer matrix into the surrounding electrolyte. In contrast, Figure 3(d) displays the fluorescence image of the same electrode after applying an oxidation potential of +0.4 V.

3.2. Voltage-Controlled Release and Uptake of Fluorescein

Figure 4(a) illustrates the electrochemically controlled release and uptake behavior of the MIP-PPy structure fabricated on a Pt electrode. After electrochemical polymerization, the electrode was immersed in DI water for 60 minutes to remove loosely bound or non-specifically adsorbed fluorescein molecules. Following this, it was positioned in the custom-designed electrochemical cell. CV measurements identified +0.4 V for uptake, within the anodic side of the PPy redox window to oxidize the polymer and insert anionic fluorescein without over-oxidation, and −0.4 V for release, on the cathodic side to expel anions and avoid the −0.7 V fluorescein reduction peak. During the first experimental cycle (Set 1, see the supplemental video), alternating voltage pulses were applied: −0.4 V for 1 second followed by +0.4 V for 4 seconds, repeated continuously for 600 seconds with a continuous flow to prevent dye accumulation. In the initial 5 seconds, a fluorescence intensity equivalent to approximately 220 nM fluorescein was detected, indicating significant initial release. This concentration decreased to ~100 nM between 100–130 seconds, and by the final 5 seconds, it dropped to ~20–25 nM, highlighting that over 90% of the initially embedded fluorescein was released. This result confirms the formation of molecular recognition cavities within the MIP-PPy matrix, allowing for selective release.

Figure 4.

(a) Fluorescein release from MIP-PPy. (b) Release from NIP-PPy. (c) MIP selectivity test with Pyranine.

To validate reuptake capability, the same MIP-PPy electrode was immersed in a 2 mM fluorescein solution in 0.1 M KCl and held at +0.4 V for 240 seconds. After rinsing in DI water for 60 minutes, the electrode underwent a second release experiment (Set 2). The identical alternating pulses were applied, resulting in a release of ~120–125 nM within the first 5 seconds, followed by a comparable decay pattern (~50 nM at 100–130 seconds and ~20–25 nM by the end), reaffirming the ~90% release efficiency of Set 1. Nearly identical results were obtained from two additional cycles (Sets 3 and 4) under identical conditions. These results confirm the reproducibility and reversibility of the MIP-PPy system and demonstrate the robust capacity of the system for selective, voltage-controlled uptake and release across multiple cycles.

In parallel, a NIP-PPy film was prepared and evaluated using the same protocol for comparison. Unlike MIP-PPy, NIP-PPy lacks specific molecular recognition sites. Prior to Set 1 (Figure 4(b), Set 1), a pre-loading step involved applying +0.4 V for 240 seconds in a 2 mM fluorescein/0.1 M KCl solution. CV confirmed similar redox potentials for NIP-PPy: +0.4 V (oxidation) and −0.4 V (reduction). During Set 1, a transient release of ~25 nM was observed within the first 100 seconds, but no further release occurred from 100–600 seconds. The lack of imprinting and nonspecific adsorption is expected to be the cause of this limited release. Additional uptake-release cycles (Sets 2–4) showed no measurable fluorescein release, confirming the lack of stable binding and controlled release in the NIP-PPy film. This contrast further validates the functionality of the MIP-PPy system.

To assess molecular selectivity, a control experiment was conducted using Pyranine (8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt), a highly anionic, fluorescent dye that shares partial structural and charge similarity with fluorescein. Pyranine is commonly used in pH sensing and bioimaging and serves as a valuable probe for distinguishing specific versus nonspecific electrostatic interactions in polyelectrolyte systems.

Figure 4(c) shows the performance of the MIP-PPy film after fluorescein removal. The electrode was immersed in a 2 mM Pyranine solution in 0.1 M KCl, and +0.4 V was applied for 240 seconds to promote uptake. Following a 60-minute DI water rinse, the alternating voltage protocol (−0.4 V for 1 s, +0.4 V for 4 s, repeated for 600 seconds) was applied. In Set 1, an initial fluorescence intensity of ~40 nM was observed in the first 30 seconds, decreasing to 20–25 nM between 100–130 seconds and to ~2–4 nM by the end. Repeating the experiment for three additional cycles (Sets 2–4) yielded similar patterns: initial release of 20–25 nM, declining to 5–10 nM mid-cycle, and <5 nM by the final time window. Compared to the robust and sustained fluorescein release in Figure 4(a), the markedly weaker Pyranine response in Figure 4(c) provides strong evidence of the MIP-PPy film’s high molecular selectivity. Despite its similar charge and fluorescence properties, Pyranine was released at much lower concentrations, likely due to nonspecific electrostatic interactions and its higher net negative charge, which may have reduced its affinity for the imprinting cavities. Collectively, these results support that template-specific uptake and release are facilitated by precise molecule recognition in the MIP-PPy structure. In contrast, both NIP-PPy and non-templated molecules exhibit minimal and non-reproducible responses, validating the effectiveness and selectivity of the imprinting process.

3.3. Effect of Electrical Pulse Duration on Release Kinetics

Figure 5 shows how varying the duration of the applied potential affects the release kinetics of the MIP-PPy device, as seen in the normalized fluorescence intensity profiles. All experiments followed a standard uptake protocol: the MIP-modified electrode was immersed in a 2 mM fluorescein solution in 0.1 M KCl and held at +0.4 V for 240 seconds, followed by a 60-minute rinse in deionized water to remove loosely bound dye. In the first experiment, a release pulse of −0.4 V for 1 second, followed by an uptake pulse of +0.4 V for 6 seconds, was repeated over a 600-second cycle. This resulted in an initial release of approximately 100 nM fluorescein in the first 20 seconds, decreasing to ~75 nM between 100 and 130 seconds, and to 20–25 nM by the end of the cycle. The release pulse duration at −0.4 V was increased to 3 seconds while the pulse period remained the same in the second experiment. The release increased significantly, reaching ~200 nM in the first 20 seconds, ~150 nM at 100–130 seconds, and ~5 nM by the final interval. These results indicate that longer release pulses enhance the efficiency of dye release. The prolonged-release pulse exhibits a sharper drop in fluorescence intensity after 100 seconds, despite the initial release appearing similar under both conditions. This demonstrates that the release kinetics in MIP-based delivery systems can be controlled by release pulse duration.

Figure 5.

Effect of release pulse duration on fluorescein release kinetics from MIP-PPy.

3.4. Time-Dependent and Sustained Release/Uptake Behavior

The time-controlled, on-demand release behavior of the MIP-PPy structure is shown in Figure 6, demonstrating its potential for programmable and stimulus-responsive drug delivery applications. The results indicate that the release of fluorescein could be selectively triggered at user-defined time intervals using electrical stimulation. The MIP-PPy electrode was subjected to alternating voltage pulses of −0.4 V for 1 second (release) and +0.4 V for 4 seconds (uptake), using the same electrochemical flow cell described previously. To maintain a clean and responsive sensing environment, a continuous upward flow of 0.1 M KCl electrolyte was maintained via a syringe pump, allowing the removal of unbound dye molecules between cycles. During the initial 240-second stimulation period, fluorescein concentration decreased from approximately 200 nM to 100 nM, confirming voltage-triggered release from the MIP matrix. Afterward, the device was left undisturbed in the same electrolyte for 5 hours, and then, the same electrochemical stimulation was applied again for another 240 seconds. Three more cycles of this procedure were carried out, each with a 5-hour break period. Consistent release profiles were observed in every cycle, confirming that the MIP structure retained functionality during extended inactive periods. A gradual reduction in maximum fluorescence intensity across cycles was attributed to cumulative dye depletion. For drug delivery systems that need to respond to physiological cues or clinical schedules with spatiotemporal accuracy, these studies show that the MIP-PPy platform allows reversible, electrically triggered molecular release at programmable time points.

Figure 6.

On-demand molecular release from MIP-PPy at scheduled time intervals.

Figure 7 further explores the long-term usage of the MIP-PPy structure under static (non-flowing) electrolyte conditions. In this setup, the syringe pump was turned off, and the electrolyte was preloaded with 50 nM fluorescein to simulate sustained analyte availability. The electrode was subjected to continuous alternating voltage pulses (−0.4 V for 1 second and +0.4 V for 4 seconds), and five sequential measurement sets (240 seconds each) were collected over a total of 1280 seconds, with 60-minute intervals between sets. Zoomed-in fluorescence traces for Sets 1–5 are shown. In the absence of intermediate rinsing and under constant dye availability, the MIP-PPy device exhibited repeatable release-uptake dynamics. In Set 1, the fluorescence intensity oscillated between ~50 and 80 nM and gradually decreased. By Sets 3 through 5, the signal stabilized within a narrower range of 50 to 75 nM, suggesting that the system had reached a dynamic equilibrium between release and reuptake. These findings confirm that the MIP-PPy structure can operate reliably over long durations, even in static conditions, maintaining functionality without externally supplied analyte. Importantly, these results highlight the unique potential of MIP-based interfaces as self-regenerating molecular delivery systems. Unlike conventional chemical stimulation platforms that rely on finite, preloaded reservoirs requiring periodic refilling, the MIP-PPy system enables local uptake and release of specific molecules at the device interface. Without the use of external reservoirs, the local concentration of a target molecule at the MIP-PPy interface can be gradually changed by adjusting voltage pulse parameters and utilizing environmental analyte availability. Applications involving chemical neurostimulation are particularly pertinent to this capability. For example, photoreceptors in the retina communicate with bipolar cells by the gradual change of neurotransmitter concentration. This biological modulation relies on a delicate balance of release and reuptake mechanisms. The MIP-PPy platform may offer a biomimetic alternative by allowing electrically controlled, bidirectional chemical modulation at the interface. Such a system could overcome the limitations of conventional reservoir-based approaches and enable closed-loop, adaptive neurochemical interfaces for brain-machine communication, targeted neuromodulation, or chemical retinal prostheses.

Figure 7.

Sustained release and uptake of fluorescein under static conditions.

3.5. Biocompatibility of Polypyrrole Films

To evaluate the biocompatibility of the electrochemically synthesized polypyrrole (PPy) films for potential bioelectronic and neurostimulation applications, two human cell lines—Huh7 (hepatoma) and SH-SY5Y (neuroblastoma)—were cultured on PPy-coated substrates. Prior to cell seeding, the PPy-coated surfaces were functionalized with fibronectin (10 μg/mL) to enhance cell adhesion. Cells were seeded at densities of either 3×10⁵ or 6×10⁵ cells per well and maintained in DMEM/F-12 supplemented with 10% fetal bovine serum (FBS) and 1% Penicillin-Streptomycin, in a 37 °C, 5% CO₂ incubator. A live/dead cell viability assay was performed 24 hours post-seeding using Calcein AM (2 μM) to stain viable cells, Ethidium homodimer-1 (EthD-1, 2 μM) for dead cells, and Hoechst 33342 for nuclear labeling. Staining reagents were diluted in Hanks’ Balanced Salt Solution (HBSS), and the cells were pre-rinsed with HBSS to remove serum residues. The prepared dye mixture was incubated with the cells for 1 hour at 37 °C, followed by replacement with fresh culture medium for 15 minutes to allow intracellular ester hydrolysis. Imaging was performed using a confocal fluorescence microscope. Figure 8(a) shows representative fluorescence images of Huh7 and SH-SY5Y cells cultured on PPy-coated surfaces, including individual and merged channels for Calcein AM (green, live cells), EthD-1 (red, dead cells), and Hoechst (blue, nuclei). Quantification revealed a high percentage of viable cells relative to total nuclei (98.5% for Huh7 and 95.4% for SH-SY5Y), with minimal dead cell staining, indicating good cytocompatibility of the PPy surfaces for both cell lines (Figure 8(b)). These results validate the prospective use of electrochemically deposited PPy films as biointerface materials in future applications, including neural prostheses, drug delivery systems, and chemically modulated neurostimulation platforms.

Figure 8.

(a) Live/dead fluorescence imaging of SH-SY5Y and Huh7 cells on PPy-coated substrates showing high biocompatibility. Scale bars: 50 μm (b) Quantification of the percentage of Calcein AM-positive live cells (green) and EthD-1 dead cells (red). Total of 569 Huh7 cells and 651 SH-SY5Y cells in 3 different areas were counted. Data are shown as Mean ±SD.

4. Conclusion

In this study, we investigated the real-time uptake and release behavior of a model molecule, fluorescein sodium salt, using molecularly imprinted polypyrrole (MIP-PPy) films. By incorporating the fluorescent dye as a template during electrochemical polymerization, we enabled direct optical tracking of molecular transport at the polymer interface, allowing for the visualization and quantification of molecule release and reuptake under voltage control. This provides insights into the dynamics of electrically gated molecular exchange. Our results demonstrate that the release kinetics can be modulated by tuning the duration of electrical pulses and that uptake-release cycles remain responsive even after extended idle periods. It was found that the MIP-PPy structure exhibited reversible release and reuptake equilibrium at static conditions. This demonstrates that continuous and controllable operation without relying on passive diffusion or preloaded reservoirs is possible. Although the materials and fabrication approach are based on established MIP-PPy systems, our work contributes a key advancement in enabling real-time, fluorescence-based monitoring of electrochemical molecular transport. This capability is particularly valuable for studying and optimizing future smart release systems and may inform the development of adaptive biointerfaces in biosensing and neuromodulation. Biocompatibility tests using SH-SY5Y neuroblastoma and Huh7 hepatoma cell lines confirmed that the PPy surfaces support viable and adherent cell cultures, validating the suitability of the interface for biological applications. This study establishes a visual and quantitative framework for examining electrically driven molecular release and reuptake, thereby setting the foundation for future research on closed-loop, chemically responsive bioelectronic systems.

Supplementary Material

Supplementary Video: Real-Time Fluorescence Imaging of Voltage-Gated Molecular Exchange in MIP-PPy Film (−0.4 V for 1 s, +0.4 V for 4 s, 1-minute Cycle)

Highlights.

• Real-time imaging reveals voltage-gated release and reuptake in MIP-PPy films.

• Electropolymerized MIP-PPy films enable the programmable transport of molecules.

• Self-regenerating uptake can remove the need for preloaded reservoirs.

• Custom flow cell enables electrical control and fluorescence monitoring in situ.

Biographies

Mayank Pandey received his B.Sc. degree in Physics from the University of Delhi, Delhi, India, in 2016. He received his M.Sc. degree in Physics from Visvesvaraya National Institute of Technology (VNIT), Nagpur, India, in 2019. He worked as a Teaching Assistant at Delhi Technological University, Delhi, India, from 2020–2021. Currently, he is pursuing his Ph.D. in Electrical Engineering at The City College of New York. His research focuses on micro-actuators and sensors, integrated micro/nano systems, electrochemical sensors, conducting polymers, and micro-fabrication and processing.

Vidhyalakshmi Acharya obtained her B.Sc. and M.Sc. degrees in Biotechnology from the University of Mysore, India, in 2014 and 2016 respectively. She then worked as a Graduate Trainee for one year at the Simons Centre for the Study of Living Machines, National Centre for Biological Sciences Bangalore involved in experimental evolution studies using yeast models. In 2018, she joined the Mechanobiology Institute, National University of Singapore for her doctoral program, focusing on investigating the live dynamics of lumenogenesis and tubulogenesis during bile canaliculi formation in hepatocytes, and completed her PhD in 2024. Currently, she is a Postdoctoral Research Fellow in the Shin Lab at the Hackensack Meridian Health Center for Discovery and Innovation, studying the role of nuclear envelope proteins in regulating liver lipid metabolism

Ji-Yeon Shin received a B.S. degree in Biology Education from Chonnam National University, a M.S. degree in Developmental Biology from the Kwangju Institute of Science and Technology, South Korea, in 1996 and 1998. She received a Ph.D. degree in Genetics and Molecular Biology from Emory University in 2006. During her doctoral degree, she studied neurogenetics and neuropathology in neurodegenerative diseases. She then obtained an additional year of training in neurogenetics as a postdoctoral scientist at UCSF. After she joined at Columbia University, she worked as a postdoctoral research scientist and associate research scientist from 2008 to 2019. In 2019, she was appointed as an Assistant Professor in Medical Sciences in the Department of Medicine. Her laboratory was moved to Hackensack Meridian Health Center for Discovery and Innovation in 2024. Her current research focuses on basic and translational studies of nuclear envelope proteins in health and diverse diseases.

Sang-Woo Seo received a B.S. degree in electrical engineering from Ajou University, a M.S. degree from the Kwangju Institute of Science and Technology, South Korea, and a Ph.D. degree in electrical engineering from the Georgia Institute of Technology, USA, in 1997, 1999, and 2003, respectively. From 2003 to 2007, he worked as a research engineer at Georgia Tech and Duke University, focusing on the development of thin-film photonic devices and integration techniques. He subsequently joined the University of California, Davis, as a research scientist, where he contributed to the development of InP-based chip-scale optical code-division multiplex access systems and arbitrary waveform generators. In 2007, he joined the Department of Electrical Engineering at the City College of New York, NY, USA, where he is currently a Professor. His research interests include the integration of heterogeneous optoelectronic devices, integrated microfluidic and photonic sensors, integrated THz systems, bioMEMS, and neuro-prosthetic interfaces.