Diamond nanowires: a novel platform for electrochemistry and matrix-free mass spectrometry

Abstract

Over the last decades, carbon-based nanostructures have generated a huge interest from both fundamental and technological viewpoints owing to their physicochemical characteristics, markedly different from their corresponding bulk states. Among these nanostructured materials, carbon nanotubes (CNTs), and more recently graphene and its derivatives, hold a central position. The large amount of work devoted to these materials is driven not only by their unique mechanical and electrical properties, but also by the advances made in synthetic methods to produce these materials in large quantities with reasonably controllable morphologies. While much less studied than CNTs and graphene, diamond nanowires, the diamond analogue of CNTs, hold promise for several important applications. Diamond nanowires display several advantages such as chemical inertness, high mechanical strength, high thermal and electrical conductivity, together with proven biocompatibility and existence of various strategies to functionalize their surface. The unique physicochemical properties of diamond nanowires have generated wide interest for their use as fillers in nanocomposites, as light detectors and emitters, as substrates for nanoelectronic devices, as tips for scanning probe microscopy as well as for sensing applications. In the past few years, studies on boron-doped diamond nanowires (BDD NWs) focused on increasing their electrochemical active surface area to achieve higher sensitivity and selectivity compared to planar diamond interfaces. The first part of the present review article will cover the promising applications of BDD NWS for label-free sensing. Then, the potential use of diamond nanowires as inorganic substrates for matrix-free laser desorption/ionization mass spectrometry, a powerful label-free approach for quantification and identification of small compounds, will be discussed.

Keywords: SALDI; diamond nanostructures; diamond nanowires; electrochemical sensing; mass spectrometry; synthetic methods.

Figure 1

Selected examples of electrochemical sensing using BDD electrodes.

Figure 2

Different morphologies and forms of diamond.

Figure 3

(A) SEM images of diamond nanowiskers formed on as-grown diamond films coated with 400 nm thick Al layer using reactive ion etching (RIE) with oxygen plasma (reprint with permission from [16]); (B) Formation of honeycomb diamond films (reprint with permission from [15]).

Figure 4

(A) Top-down etching process of CVD diamond using seed particles; (B) SEM image of diamond cylinders obtained using oxygen reactive ion etching (RIE) for 60 min through a 1 µM SiO₂ particle array (reprint with permission from [18]); (C) SEM image of vertically aligned diamond nanowires using diamond nanoparticles as masks (with courtesy of C. Nebel).

Figure 5

(A) Top-down etching process of CVD diamond without mask; (B) SEM and HR-TEM images, and XPS survey spectrum of boron-doped diamond nanowires synthesized through maskless technique (reprint with permission from [26]).

Figure 6

(A) Schematic diagram of the bottom-up fabrication of cylindrical diamond wires in porous alumina template (left) and the corresponding SEM images (reprint with permission from [28]); (B) TEM (left) and HR-TEM (right) images of diamond rods grown on carbon nanotubes (reprint with permission from [29]); (C) SEM image of diamond coated silicon nanowires (reprint with permission from [10]).

Figure 7

Applications of diamond nanowires.

Figure 8

Diamond nanowires for DNA sensing: Preferential linking of phenyl aryl and DNA molecules to the tip of the wires (reprint with permission from [23,44]).

Figure 9

Diamond nanowires for enzyme-free glucose and tryptophan sensing: (A) SEM image of long diamond nanowires produced through mask-less RIE of CVD diamond films and linear sweep voltammogram recorded in 2 mM glucose solution (0.1 M NaOH) for BDD (black) and long BDD NWs (blue) (reprint with permission from [12]); (B) SEM image of short diamond nanowires together with differential pulse voltammograms for different concentrations of tryptophan and the corresponding calibration curve (reprint with permission from [25]).

Figure 10

Diamond nanowires for immunosensing. (A) (a) fabrication method of polymer coated BDD NWs (PPA BDD NWs) with chelated Cu²⁺ ions and subsequently modified with histidine-terminated target; (b) SEM images of BDD NWs after electrochemical deposition of carboxylic acid-terminated poly(pyrrole) (100 mM) in TBATFB (0.1 M)/ acetonitrile solution at E = +1.2 V for different deposition charges (2, 11, 23 mC·cm⁻²) (reprint with permission from [39]); (B) Fabrication method of Ni NPs modified diamond nanowires (a), SEM image of Ni NPs modified diamond nanowires (b), CV of Ni NPs-BDD NWs in 0.1 M NaOH (c), Calibration curve for IgG (d) (reprint with permission from [40]).

Figure 11

Diamond nanowires as inorganic matrices for SALDI. (A) Schematic presentation of the principle of surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) with analyte deposition on SALDI based nanostructures such as diamond nanowires; (B) OTS modified diamond nanowires (a) and contact angle values before and after UV/O₃ treatment (b); (C) LDI-MS spectra on BDD NWs of various compounds (Histidine, m/z 156, 1 pmol; Betaine m/z 118, 1 pmol; Cortisone m/z 361, 1 pmol and verapamil m/z 455, 2 pmol and 200 zmol (inset)); (D) LDI-MS detection of a peptide mixture ([Des-Arg¹]-bradykinin m/z 904 (50 fmol/µL), angiotensin I m/z 1296 (50 fmol/µL), [Glu¹]-fibrinopeptide B m/z 1570 (50 fmol/µL), neurotensin m/z 1673 (10 fmol/µL)) on BDD NWs and BDD films (reprint with permission from [27]).

Figure 11

Diamond nanowires as inorganic matrices for SALDI. (A) Schematic presentation of the principle of surface-assisted laser desorption/ionization mass spectrometry (SALDI-MS) with analyte deposition on SALDI based nanostructures such as diamond nanowires; (B) OTS modified diamond nanowires (a) and contact angle values before and after UV/O₃ treatment (b); (C) LDI-MS spectra on BDD NWs of various compounds (Histidine, m/z 156, 1 pmol; Betaine m/z 118, 1 pmol; Cortisone m/z 361, 1 pmol and verapamil m/z 455, 2 pmol and 200 zmol (inset)); (D) LDI-MS detection of a peptide mixture ([Des-Arg¹]-bradykinin m/z 904 (50 fmol/µL), angiotensin I m/z 1296 (50 fmol/µL), [Glu¹]-fibrinopeptide B m/z 1570 (50 fmol/µL), neurotensin m/z 1673 (10 fmol/µL)) on BDD NWs and BDD films (reprint with permission from [27]).