Graphene transistors for real-time monitoring molecular self-assembly dynamics

Understanding the dynamics of molecular self-assembly on surfaces is crucial for engineering materials with predictable structures and properties. Traditional methods like scanning probe microscopy (SPM), particularly scanning tunneling microscopy (STM), offer high spatial resolution but are limited in their temporal resolution and ability to characterize macroscopic self-assembly involving billions of molecules. STM's temporal resolution is typically limited to tens of frames per second, or even slower when visualizing molecular assemblies, providing a limited view of the overall dynamics. While electronic devices have been used to monitor single-molecule reactions and DNA hybridization in real time, their application to the complex ensemble process of on-surface self-assembly has been lacking. This research aims to address this gap by exploring the use of graphene field-effect transistors (GFETs) as sensitive electrical detectors to monitor the dynamics of on-surface self-assembly.

The literature extensively details the use of scanning probe microscopy, especially STM, to image the structures resulting from molecular self-assembly on surfaces. Studies have shown the ability to visualize the kinetics of nucleation and rearrangement in supramolecular adlayers, including light-responsive assemblies of photochromic molecules with sub-nanometer resolution. However, the limited field of view of STM restricts its ability to describe the population dynamics across macroscopic distances. The existing literature also highlights the challenges in achieving high temporal resolution with STM when monitoring molecular assemblies, often limited to milliseconds or seconds. Therefore, there's a need for alternative techniques offering both high sensitivity and ultrafast temporal resolution for monitoring the dynamic aspects of molecular self-assembly on surfaces.

The researchers employed a spiropyran (SP) derivative with an octadecyl side chain, which promotes self-assembly. UV irradiation (365 nm) converts the SP isomer to a metastable merocyanine (MC) isomer, which self-assembles at the graphene/solution interface. The photoisomerization process was first characterized optically by monitoring the absorbance change at 590 nm. STM was used to visualize the formation and desorption of the MC assembly at the interface between highly oriented pyrolytic graphite (HOPG) and the SP solution. For the electrical measurements, GFETs were fabricated using both mechanically exfoliated and CVD graphene. A drop of SP solution was placed on the graphene surface, and the current was measured while UV light was switched on and off. The change in current reflects the adsorption and desorption of the MC adlayer, acting as a light-induced gate terminal. The study also investigated the concentration dependence of self-assembly dynamics by varying the initial SP concentration and measuring both absorbance and current changes.

The study revealed that the MC isomer, generated by UV irradiation, forms a highly ordered lamellar assembly on the HOPG surface, which is readily observed using STM. Importantly, this assembly is metastable and desorbs upon the thermal relaxation of MC back to the SP isomer. Graphene transistors exhibited high sensitivity to the formation and desorption of the MC adlayer, enabling real-time monitoring of the self-assembly dynamics. The change in drain-source current (IDS) accurately reflects the surface coverage of the MC molecules. Analysis of the current changes during UV irradiation and subsequent relaxation revealed time constants characterizing both the formation and desorption processes. The time constants for self-assembly formation showed a strong dependence on the initial SP concentration, while the photoisomerization time constant in solution remained relatively constant, highlighting the ability of graphene devices to distinguish between interfacial and bulk processes. The study demonstrated that the electrical response of the graphene transistor was consistent across different device sizes, suggesting scalability to smaller areas, potentially approaching the spatial resolution of STM. The study demonstrates that the graphene-based method allows to clearly distinguish the dynamics of formation of the self-assembled adlayer at the interface from the dynamics of isomerization in the bulk solution.

The results demonstrate that graphene transistors provide a powerful and sensitive method for real-time monitoring of molecular self-assembly dynamics at the solid-liquid interface. This electrical readout method surpasses the limitations of STM in terms of temporal resolution and the number of molecules monitored. The ability to differentiate between the kinetics of photoisomerization in solution and the formation of the self-assembled adlayer highlights the high surface sensitivity of graphene. The strong dependence of the self-assembly formation time constant on the SP concentration suggests that the process is governed by a critical concentration of MC molecules near the surface. The successful demonstration of this approach opens new avenues for investigating complex molecular self-assembly processes and provides a platform for advanced sensing applications.

This study successfully demonstrated the use of graphene transistors as highly sensitive detectors for real-time monitoring of molecular self-assembly dynamics at the solid-liquid interface. The method offers high sensitivity, practicality, and the potential for ultrafast time response, enabling the study of rapidly evolving processes. The ability to distinguish between interfacial and bulk processes, combined with the potential for miniaturization, makes this technique a valuable tool for investigating a range of complex self-assembly phenomena and holds significant promise for future applications in (bio)diagnostics and (bio)chemical sensing. Future research could explore the application of this technique to other self-assembly processes and the development of even faster graphene devices to achieve nanosecond-scale resolution.

The current study focused on a specific photochromic molecule and experimental conditions. The generalizability of the findings to other molecules and self-assembly systems needs further investigation. While the study demonstrated the scalability of the graphene transistor approach, the actual implementation at the nanoscale needs further optimization. The time resolution in this study was limited by the experimental setup; however, faster graphene devices could potentially achieve nanosecond-scale resolution. Finally, the specific solvent used might influence the self-assembly dynamics and further studies are needed to understand the role of the solvent in the self-assembly kinetics.