A Versatile and Simple Approach to Electrochemical Colloidal Probes for Direct Force Measurements

Pt-Black-Modified (Hemi)spherical AFM Sensors: In Situ Imaging of Light-Driven Hydrogen Peroxide Evolution

In this work, we present (hemi)spherical atomic force microscopy (AFM) sensors for the detection of hydrogen peroxide. Platinum-black (Pt-B) was electrodeposited onto conductive colloidal AFM probes or directly at recessed microelectrodes located at the end of a tipless cantilever, resulting in electrocatalytically active cantilever-based sensors that have a small geometric area but, due to the porosity of the films, exhibit a large electroactive surface area. Focused ion beam-scanning electron microscopy tomography revealed the porous 3D structure of the deposited Pt-B. Given the accurate positioning capability of AFM, these probes are suitable for local in situ sensing of hydrogen peroxide and at the same time can be used for (electrochemical) force spectroscopy measurements. Detection limits for hydrogen peroxide in the nanomolar range (LOD = 68 ± 7 nM) were obtained. Stability test and first in situ proof-of-principle experiments to achieve the electrochemical imaging of hydrogen peroxide generated at a microelectrode and at photocatalytically active structured poly(heptazine imide) films are demonstrated. Force spectroscopic data of the photocatalyst films were recorded in ambient conditions, in solution, and by applying a potential, which demonstrates the versatility of these novel Pt-B-modified spherical AFM probes.

Electrogelation: Controlled Fast Formation of Micrometer-Thick Films from Low-Molecular Weight Hydrogelators

The controlled electrochemical deposition of hydrogels from low-molecular weight hydrogelators (LMWHGs) allows for the defined formation of thin films on electrodes. Here, the deposition of fibrillar networks consisting of N,N',N″-tris(4-carboxyphenylene)-1,3,5-benzenetricarboxamide (BTA) onto ultraflat gold electrodes has been studied. This process, also termed electrogelation, is based on a local change in the pH due to electrolysis of water at the electrode. The protonation of the BTA sodium salt leads to self-assembly into supramolecular fibrillar structures mainly via hydrogen bonding of the uncharged molecules. The resulting hydrogel film was characterized in terms of its thickness by atomic force microscopy (AFM). Two different AFM-based techniques have been used: ex situ imaging of dried films and in situ nanoindentation of the hydrated hydrogel films. The deposition process was studied as a function of gelator concentration, applied potential, and gelation time. These parameters allow control of the film thickness to a high degree of accuracy within a few tenths of nanometers. Film formation takes place in a few seconds at moderate applied potentials, which is beneficial for biomedical applications. The results obtained for the BTA presented here can be transferred to any type of pH-responsive LMWHG and many reversibly formed hydrogel films.

An Integrated, Exchangeable Three-Electrode Electrochemical Setup for AFM-Based Scanning Electrochemical Microscopy

Scanning electrochemical microscopy (SECM) is a versatile scanning probe technique that allows monitoring of a plethora of electrochemical reactions on a highly resolved local scale. SECM in combination with atomic force microscopy (AFM) is particularly well suited to acquire electrochemical data correlated to sample topography, elasticity, and adhesion, respectively. The resolution achievable in SECM depends critically on the properties of the probe acting as an electrochemical sensor, i.e., the working electrode, which is scanned over the sample. Hence, the development of SECM probes received much attention in recent years. However, for the operation and performance of SECM, the fluid cell and the three-electrode setup are also of paramount importance. These two aspects received much less attention so far. Here, we present a novel approach to the universal implementation of a three-electrode setup for SECM in practically any fluid cell. The integration of all three electrodes (working, counter, and reference) near the cantilever provides many advantages, such as the usage of conventional AFM fluid cells also for SECM or enables the measurement in liquid drops. Moreover, the other electrodes become easily exchangeable as they are combined with the cantilever substrate. Thereby, the handling is improved significantly. We demonstrated that high-resolution SECM, i.e., resolving features smaller than 250 nm in the electrochemical signal, could be achieved with the new setup and that the electrochemical performance was equivalent to the one obtained with macroscopic electrodes.

Electrochemical grippers based on the tuning of surface forces for applications in micro- and nanorobotics

Existing approaches to robotic manipulation often rely on external mechanical devices, such as hydraulic and pneumatic devices or grippers. Both types of devices can be adapted to microrobots only with difficulties and for nanorobots not all. Here, we present a fundamentally different approach that is based on tuning the acting surface forces themselves rather than applying external forces by grippers. Tuning of forces is achieved by the electrochemical control of an electrode's diffuse layer. Such electrochemical grippers can be integrated directly into an atomic force microscope, allowing for 'pick and place' procedures typically used in macroscopic robotics. Due to the low potentials involved, small autonomous robots could as well be equipped with these electrochemical grippers that will be particularly useful in soft robotics as well as nanorobotics. Moreover, these grippers have no moving parts and can be incorporated in new concepts for actuators. The concept can easily be scaled down and applied to a wide range of objects, such as colloids, proteins, and macromolecules.

Studies of probe tip materials by atomic force microscopy: a review

As a tool that can test insulators' surface morphology and properties, the performance index of atomic force microscope (AFM) probes is the most critical factor in determining the resolution of microscopy, and the performance of probes varies in various modes and application requirements. This paper reviews the latest research results in metal, carbon nanotube, and colloidal probes and reviews their related methods and techniques, analyses the advantages and disadvantages of the improved probes compared with ordinary probes by comparing the differences in spatial resolution, sensitivity, imaging, and other performance aspects, and finally provides an outlook on the future development of AFM probes. This paper promotes the development of AFM probes in the direction of new probes and further promotes the broader and deeper application of scanning probe microscope (SPM).