Objectives

To investigate the formation of hydroxyapatite (HAP) from calcium silicate and the deposition of calcium silicate onto sound and acid eroded enamel surfaces in order to investigate its repair and protective properties.

Methods

Calcium silicate was mixed with phosphate buffer for seven days and the resulting solids analysed for crystalline phases by Raman spectroscopy. Deposition studies were conducted on bovine enamel surfaces. Acid etched regions were produced on the enamel surfaces using scanning electrochemical cell microscopy (SECCM) with acid filled pipettes and varying contact times. Following treatment with calcium silicate, the deposition was visualised with FE-SEM and etch pit volumes were measured by AFM. A second set of bovine enamel specimens were pre-treated with calcium silicate and fluoride, before acid exposure with the SECCM. The volumes of the resultant acid etched pits were measured using AFM and the intrinsic rate constant for calcium loss was calculated.

Results

Raman spectroscopy confirmed that HAP was formed from calcium silicate. Deposition studies demonstrated greater delivery of calcium silicate to acid eroded than sound enamel and that the volume of acid etched enamel pits was significantly reduced following one treatment (p<0.05). In the protection study, the intrinsic rate constant for calcium loss from enamel was 0.092±0.008 cm/s. This was significantly reduced, 0.056±0.005 cm/s, for the calcium silicate treatments (p<0.0001).

Conclusions

Calcium silicate can transform into HAP and can be deposited on acid eroded and sound enamel surfaces. Calcium silicate can provide significant protection of sound enamel from acid challenges.

Clinical Significance

Calcium silicate is a material that has potential for a new approach to the repair of demineralised enamel and the protection of enamel from acid attacks, leading to significant dental hard tissue benefits.

Highly doped, boron doped diamond (BDD) is an electrode material with great potential, but the fabrication of suitable electrodes in a variety of different geometries both at the macro and microscale, with an insulating material that does not compromise the material properties of the BDD, presents technical challenges. In this Technical Note a novel solution to this problem is presented, resulting in the fabrication of co-planar macro- and microscale BDD electrodes, insulated by insulating diamond, at the single and multiple, individually addressable level. Using a laser micromachining approach, the required electrode(s) geometry is machined into an insulating diamond substrate, followed by overgrowth of high quality polycrystalline BDD and polishing to reveal ~ nm roughness, co-planar all-diamond structures. Electrical contacting is possible using both top and bottom contacts, where the latter are defined using the laser to produce non-diamond-carbon (NDC) in the vicinity of the back side of the BDD. We present the fabrication of individually addressable ring, band and disk electrodes with minimum, reproducible controlled dimensions of 50 µm (limited only by the laser system employed). The pBDD grown into the insulating diamond recesses is shown to be free from NDC and possesses excellent electrochemical properties, in terms of extended solvent windows, electrochemical reversibility and capacitance.

Thermoelectrochemical experiments can reveal significant information about electrochemical processes compared to ambient only measurements. Typical thermoelectrochemistry is performed using resistively heated wires 10 or laser heated electrodes, both of which can suffer drawbacks associated with the electrode material employed. Boron doped diamond (BDD) is ideal for thermoelectrochemical investigations due to its extremely high thermal conductivity and diffusivity, extreme resistance to thermal ablation (can withstand laser power densities, Pd, of GW cm-2 for 15 nanosecond pulses) and excellent electrochemical properties (low background currents and wide potential window). In this paper we describe the use of a pulsed laser technique to heat the rear of a 1 mm diameter conducting BDD disc electrode, which drives electrochemical solution reactions at the front face. Maximum average electrode temperatures of 90.0 oC were recorded experimentally and confirmed by finite element modelling (FEM). The effect of laser pulsed heating (maximum 3.8 kW cm^-220 ; 10 ms on and 90 ms off) on the cyclic voltammetric response of two fast (reversible) outer sphere electron transfer redox mediators (Ru(NH₃)₆^3+/2+ and IrCl₆^2-/3-) are investigated. Inparticular, we observe pulsed increases in the current, which increase with increasing Pd. The potential of the peak current is shifted positively for the Ru(NH₃)₆^3+/2+ couple (in accordance with a positive temperature coefficient, β, +0.68 mV K^-125 ) and negatively for the IrCl₆^3-/2- couple (β = -0.48 mV K^-1). Scanning backwards, in contrast to that observed for a macrodisc electrode in ambient solution, a cathodic peak is again observed for Ru(NH₃)₆^3+/2+ and an anodic peak for IrCl₆^3-/2- couple. We attribute this response to the entropy of the redox reaction and the time-dependant change in mass transport due to the
30 induced thermal gradients at the electrode/electrolyte interface. The observed responses are in qualitative agreement with FEM simulations.

Double potential step chronoamperometry (DPSC) is demonstrated as a technique for investigating partitioning between a solute in aqueous solution and non-polar oil droplet(s) immobilised at an electrode. Here a species in aqueous solution which does not partition into the oil phase is converted at the electrode surface into another species which either does not or does partition into the oil drop. The first case is investigated experimentally by considering generation of the ionic redox species, FcTMA²⁺ from FcTMA⁺, while the second case is exemplified by studies of Br₂ generation from Br⁻. The case of molecular partitioning at the three phase interface has received little attention hitherto. To maintain oil droplet stability a boron-doped diamond electrode is employed functionalised with Pt nanoparticles to impart electrocatalytic activity on the electrode towards Br₂ production. An arrangement is utilised where the droplet(s) sit(s) on (but does not cover) the electrode surface. We show both experimentally and through finite element simulation how the charge–time profile for the generation and collection of electroactive species can be used to obtain information on the extent of partitioning and how this is affected by factors such as the number and size of droplets. Finally, we highlight the suitability of this approach for investigating reactions which take place within the droplet.

The development of a novel analytical technique, electrochemical X-Ray fluorescence (EC-XRF) is described and applied to the quantitative detection of heavy metals in solution, achieving sub-ppb limits of detection (LOD). In EC-XRF electrochemical preconcentration of a species of interest onto the target electrode is achieved by cathodic electrodeposition. Unambiguous elemental identification and quantification of metal concentration is then made using XRF. This simple electrochemical preconcentration step improves the LOD of energy dispersive XRF by over four orders of magnitude (for similar sample preparation timescales). Large area free-standing boron doped diamond grown using microwave plasma chemical vapor deposition techniques is found to be ideal as the electrode material for both electrodeposition and XRF due to its wide solvent window, transparency to the XRF beam and ability to be produced in mechanically robust thin film form. During electrodeposition it is possible to vary both the deposition potential (Edep) and deposition time (tdep). For the metals Cu2+ and Pb2+ the highest detection sensitivities were found for Edep = -1.75 V and tdep = 4000 s with LODs of 0.05 ppb and 0.04 ppb achieved, respectively. In mixed Cu2+/Pb2+ solutions EC-XRF shows that Cu2+ deposition is unimpeded by Pb2+, across a broad concentration range, but this is only true for Pb2+ when both metals are present at low concentrations (10 nM); boding well for trace level measurements. In a dual mixed metal solution EC-XRF can also be employed to either selectively deposit the metal which has the most positive formal reduction potential or exhaustively deplete it from solution, enabling uninhibited detection of the second metal.

The requirement to separate topographical effects from surface electrochemistry information is a major limitation of scanning electrochemical microscopy (SECM). With many applications of SECM involving the study of (semi)conducting electrode surfaces, the hybridisation of SECM with scanning tunnelling microscopy (STM) or a surface conductance probe would provide the ultimate topographical imaging capability to SECM, but previous attempts are limited. Here, the conversion of a general scanning electrochemical probe microscopy (SEPM) platform to facilitate contact electrical conductance (C)‐ and electron tunnelling (T)‐SECM measurements is considered. Measurements in air under ambient conditions with a Pt/Ir wire tip are used to assess the performance of the piezoelectric positioning system. A hopping‐mode imaging protocol is implemented, whereby the tip approaches the surface at each pixel until a desired current magnitude is exceeded, and the corresponding z position (surface height) is recorded at a set of predefined xy coordinates in the plane of the surface. At slow tip approach rates, the current shows an exponential dependence on tip‐substrate distance, as expected for electron tunnelling. For measurements in electrochemical environments, in order to overcome well‐known problems with leakage currents at coated‐wire tips used for electrochemical STM, Pt‐sensitised carbon nanoelectrodes are used as tips. The hydrogen evolution reaction on 2D Au nanocrystals serves as an exemplar system for the successful simultaneous mapping of topography and electrochemical activity.