Light-addressable potentiometric sensors (LAPS)¹ and scanning photo-induced impedance microscopy (SPIM)², both of which are based on photocurrent measurements at electrolyte /insulator /semiconductor (EIS) field-effect structures can be used to detect local surface electrical potentials and impedance, respectively. While LAPS measures local potential changes by recording the shift of the photocurrent-voltage (I-V) curve along the voltage axis, SPIM records changes in the maximum photocurrent while the EIS structure is biased towards inversion. Both techniques have been extensively applied for chemical ion sensing, biosensing, and the investigation of cell metabolism, extracellular potentials, cell surface charge and impedance. Importantly, they have been reported to be highly sensitive to the surface contamination on self-assembled organic monolayers³.

In this work, a copper (I) catalyzed azide alkyne cycloaddition (CuAAC) reaction, combined with microcontact printing was employed successfully to pattern alkyne-terminated self-assembled organic monolayer modified silicon surfaces (Figure 1a). Despite the absence of a copper peak in XPS spectra, copper contamination was found and visualized using LAPS and SPIM, after the ‘click’ modified silicon surfaces were rinsed with hydrochloric acid solution (Figure 1b), which has been frequently used to remove copper residues in the past. Different strategies for avoiding copper contamination, including the use of bulky chelators for the copper (I) catalyst and rinsing with different reagents, were tested. Only cleaning of the silicon surfaces with an ethylenediaminetetraacetic acid (EDTA) solution after the ‘click’ modification proved to be an effective method as confirmed by LAPS and SPIM results, which showed the desirable electrical properties (surface charge and impedance) of an organic monolayer (Figure 1c).

1. Hafeman, D. G.; Parce, J. W.; McConnell, H. M., LIGHT-ADDRESSABLE POTENTIOMETRIC SENSOR FOR BIOCHEMICAL SYSTEMS. Science 1988,240, (4856), 1182-1185.

2. Krause, S.; Talabani, H.; Xu, M.; Moritz, W.; Griffiths, J., Scanning photo-induced impedance microscopy - an impedance based imaging technique. Electrochim. Acta 2002,47, (13-14), 2143-2148.

3. Wang, J.; Wu, F.; Watkinson, M.; Zhu, J. Y.; Krause, S., "Click" Patterning of Self-Assembled Monolayers on Hydrogen-Terminated Silicon Surfaces and Their Characterization Using Light-Addressable Potentiometric Sensors. Langmuir 2015, 31, (35), 9646-9654.