Advanced electroanalytical chemistry at nanoelectrodes

Abstract

Nanoelectrodes, with dimensions below 100 nm, have the advantages of high sensitivity and high spatial resolution. These electrodes have attracted increasing attention in various fields such as single cell analysis, single-molecule detection, single particle characterization and high-resolution imaging. The rapid growth of novel nanoelectrodes and nanoelectrochemical methods brings enormous new opportunities in the field. In this perspective, we discuss the challenges, advances, and opportunities for nanoelectrode fabrication, real-time characterizations and high-performance electrochemical instrumentation.

Fig. 1. Different fabrication methods for nanoelectrodes. (A) Photolithographic fabrication process for a gold nanoband electrode with (i) spin-coating SU-8 on a silicon wafer, (ii) UV exposure, (iii) SU-8 development, and (iv) Cr and Au deposition onto SU-8. (B) Reflected light micrograph of nine embedded annular nanoband electrode arrays. Insert: an SEM image of a single pore array with 11 × 11 pores. (C) The embedded annular nanoband electrode array sample at a 15° tilt with FIB milling. (D) Magnified view of the arrays at a 52° tilt. (E) The Pt nanoelectrode produced via the electrochemical etching method. A Pt (radius 0.125 mm) ring was covered with a film of etching solution, this film exists stably because of the surface tension. An AC voltage of 2 V was applied between the Pt wire and the Pt ring. The etching procedure was completed when the current dropped to zero. (F) The fabrication process of carbon nanopipette electrodes by chemical vapour deposition. (i) The pipette is pulled to the nanoscale; (ii) chemical vapour deposition of carbon; (iii) the quartz/glass at the tip of nanopipette is etched to acquire a carbon tip. (G) An SEM image of the tip profile of a carbon nanopipette electrode. (H) SEM images of carbon nanopipette electrodes with a tip diameter of 50 nm (right) and 365 nm (left). (I) Fabrication of a Pt nanoelectrode via the laser-assisted wire pulling method. The nanoelectrode fabrication procedure from left to right: the insertion of the Pt-wire into the glass capillary; the cylindrical melting of the quartz glass; the simultaneous pulling of the quartz capillary and Pt-wire; the contacting of the Pt-wire by means of a Cu-wire and a Ag-filled epoxy glue; and the polishing of the obtained nanoelectrode. (J) The experimental conditions for nanoelectrode polishing from left to right: polishing by rotating the nanoelectrode on a polishing plate in a drop of water or alumina suspension; precession of the fragile electrode tip after pressing the electrode tip onto the polishing plate leads to a conical polishing of the insulating glass sheath. Adapted with permission from ref. 22 Copyright (2012) American Chemical Society. Adapted with permission from ref. 24 Copyright (2001) American Chemical Society. Adapted with permission from ref. 38 Copyright (2015) American Chemical Society. Adapted with the permission of ref. 20 and 45 from John Wiley and Sons.

Fig. 2. Characterization of nanoelectrodes. (A) SEM image of a ∼110 nm-diameter Pt nanoelectrode; (B) TEM image of a 1.5 nm radius Pt nanowire sealed in an SiO₂ capillary; (C) TEM image of a 3 nm radius Pt nanoelectrode sealed in an SiO₂ capillary. Noncontact AFM image of a recessed Pt nanoelectrode in air in (D) 3D and (E) 2D. (F) Steady-state voltammogram of 1.2 mM FcCH₂OH. The potential sweep rate was v = 50 mV s^–1. Adapted with permission from ref. 16 Copyright (2009) American Chemical Society. Adapted with permission from ref. 55 Copyright (2013) American Chemical Society. Adapted with permission from ref. 72 Copyright (2012) American Chemical Society.

Fig. 3. (A) Circuit diagram of a low-current amplifier. During experiments, the bias voltage is set by applying a reference voltage to the non-inverting input of the trans-impedance amplifier (TIA). The current signal is transferred to a voltage signal through the TIA. Then, a unity gain differential amplifier is used to deduct the reference voltage from the signal. To lower the noise, a low-pass filter is always used before the signal is transferred to the digital signal by the analog-to-digital convertor (ADC). (B) Chronoamperometric curves of single 4-mercaptobenzene-1,2-diol@AuNP collisions on a carbon-fibre ultramicroelectrode (UME) recorded at a filter frequency of 1 kHz, 2 kHz and 5 kHz, respectively. The experiments were carried out in a 15 mM phosphate buffer (pH 7.0) containing 5 mM NADH. Adapted with permission from ref. 74 Copyright (2016) American Chemical Society.

Fig. 4. (A) TEM image of a single Au nanoparticle immobilized on a Pt nanoelectrode. (B) Cyclic voltammetry of a bare 7 nm diameter Pt nanoelectrode (black), a 14 nm Au single nanoparticle immobilized Pt nanoelectrode (SNPE), an 18 nm Au SNPE (green) and a 24 nm Au SNPE (blue) in an oxygen-saturated 0.10 M KOH solution. TEM images of (C) a carbon nanopipette electrode and (D) a carbon nanopipette electrode with a 20 nm AuNP attached to its tip. (E) The current transients of 80 nm collisions of individual AgNPs with a nominal diameter on the Au UME (diameter of UME: 12.5 μm) at an applied potential of +0.6 V vs. Ag/AgCl QRE. (i) i–t transients response; (ii and iii) close-ups of the representative spike clusters shown by crosses with different colours. (iv) Enlarged sections of the red and blue transient in (iii). Adapted with permission from ref. 48 Copyright (2010) American Chemical Society. Reproduced from ref. 93 with permission from the Royal Society of Chemistry. Adapted with permission from ref. 73.

Fig. 5. Studying the single HRP with a BLM-Au nanoelectrode. (A) Cyclic voltammograms of the BLM-Au nanoelectrode (1) and HRP-embedded BLM-Au nanoelectrode (2) in a solution of 20 mM H₂O₂ and PBS buffer (pH: 7.4). (B) Chronoamperometric response of HRP collision into the BLM-Au electrode. Reproduced by permission of ref. 6 from The Royal Society of Chemistry.

Fig. 6. (A) Current responses from a carbon fibre nanotip electrode pushed against a PC12 cell are shown (i) without breaking into the cytoplasm and (ii) inserted into a PC12 cell. (B) A schematic of the nanoelectrode incorporated nanokit for single cell analysis. (C) Bright-field image of a nanokit electrode inserted into the cell. (D) Charges of the nanokit electrode before (a) and after (b) being inserted into the cell. Reproduced by permission of ref. 18 from Copyright (2016) National Academy of Sciences. Reproduced by permission of ref. 95 from John Wiley and Sons.

Yi-Lun Ying

Zhifeng Ding

Dongping Zhan

Yi-Tao Long