# Diamonds for Life: Developments in Sensors for Biomolecules
## 4 Diamond-Based Electrodes
### Dopamine Detection  
Voltammetric detection of dopamine was achieved with a micro-electrode prepared from a BDD film and modified by a secondary deposition process (see details in Section 2) to exhibit an external layer of nano-sized undoped crystalline diamond. This strategy helped to narrow the oxidation peak of dopamine, making it discernible from the signals of two common interferents, ascorbic acid and uric acid [13]. Note that, when the diamond electrode was not treated (with the second deposition) to feature the nanocrystals, the peaks of these two interferents overlapped with that of dopamine.  
An alternative strategy to detect dopamine in solutions with other contaminants resorted to a BDD electrode deposited on a microfiber and coated with a polymeric layer of oxidised polypyrrole (see manufacturing details in Section 3.2.2) [35; 36]. The presence of the oxidised polypyrrole coating helped to reduce the interference originated from ascorbic acid and 3,4-dihydroxyphenylacetic acid (DOPAC), a common metabolite of dopamine. This behaviour is associated with the abundance of negatively charged carboxylate groups (from the oxidised polypyrrole coating) on the electrode surface that attracts dopamine cations while repelling the negatively charged ascorbate anions. Even when an excess of ascorbic acid (0.2 mM) was present in regard to the target analyte, dopamine (0.05 mM), voltammograms showed two separate peaks, one for dopamine at 0.5 V and another for ascorbate at around 0.3 V (Figure 5) [35]. The limit of detection for dopamine was 0.1 nM at S/N = 3 (S/N is the signal-to-noise ratio), and the linear dynamic range laid between 0.5 nM and 100 uM. The amperometric response for 0.5 nM dopamine showed high stability (relative standard deviation of 5.4% for \(n=5\)) and good reproducibility (within \(\pm\) 6.2% for \(n=10\), with a solution of 1 nM of dopamine). Furthermore, the response remained the same during measurements conducted over a period of seven days, with no variation in the efficiency of rejection of the interferents (ascorbic acid and DOPAC).  
Sensitivity to dopamine in matrices containing complex backgrounds can be further increased using coupled systems. In one of such report, amperometric detection of dopamine using a BDD microelectrode was coupled with a pretreatment using flow injec  
Figure 5: Cyclic voltammograms, at a potential scan rate of 100 mV-s\({}^{-1}\), of a mixed solution containing 0.2 mM of ascorbic acid and 0.05 mM dopamine in 0.2 M phosphate buffer (pH 7): (**a**) bare BDD electrode and (**b**) oxidised polypyrrole film-coated BDD electrode. Reprinted with permission from [PERSON] et al. [36]. Copyright 2004 Wiley-VCH Verlag Gmbh & Co. KGaA, Weinhelm.  
tion analysis [45]. The system detected dopamine with good reliability and reproducibility at concentrations as low as 6.0 \(\upmu\)M (the lowest tested concentration) even when the analyte was dissolved in complex matrices such as river water and human serum. In another report, electrochemical detection of dopamine using a BDD microelectrode was coupled with capillary electrophoresis [48]. The system allowed the separation and analysis of dopamine in a mixture containing dopamine, catechol, and ascorbic acid (all at concentrations of 10 mM in phosphate buffer at pH = 6). The system performance was evaluated at different applied potentials, showing that the background current (\(\sim\)100 pA) was very stable with time and good reproducibility of the times of elution and detection (peak current or area) for each of the components of the mixture. The response had a precision better than 4.1%, a linear response between 0.1 and 1 mM, and a limit of detection of 1.7 fM.