Chloroplatinic acid

The widespread interest in detection of NO (nitric oxide) and due to its diverse biological roles has generated a significant demand for analytical techniques capable of its measurement and quantification [26 (link), 40 , 41 (link)]. However, such measurements can be difficult due to NO's widely ranging concentrations and stability. It is well known that in the human body, the effect of NO is dependent on its concentration, varying anywhere from sub-nanomolar to micromolar levels [41 (link)]. Furthermore, NO has a short half-life in vivo (typically < 10 s) due to its fast reactivity with oxygen, free radicals, thiols, and hemes [41 (link)-43 ]. An analytical tool to effectively detect NO demands a wide dynamic range, sufficient sensitivity, and fast response time. The method must also be selective toward NO over interfering species that arise from complex biological samples and NO derivatives. Electrochemistry offers many of these requirements for near real-time detection, with simple modifications to the electrode enabling sensitive and selective NO detection [26 (link), 40 ].
To make an electrode more sensitive for NO, many groups have used platinized electrodes [25 (link), 27 (link), 42 ]. In this process a chloroplatinic acid solution (containing a small amount of lead acetate) is used to electrochemically deposit black particles of platinum onto the working electrode. The primary benefit of platinized electrodes is the increased surface area, with a rough initial surface being key to Pt-black adhesion [24 ]. Many different techniques like thermal etching, sandblasting, and etching with aqua regia have been previously used to increase the roughness of the initial electrode [23 ]. To our advantage, pillar arrays already exhibit this characteristic due to the electrodeposition of gold (see Figure 1E for a detailed image of the surface area). The number of deposition cycles needed for optimal NO signal enhancement was first investigated. Repeated injections of a 70 μM NO solution (made from a NONOate salt) were made over a bare pillar array (4 electrodes with 20 μm tall pillars), a pillar array with 1 deposition cycle of Pt-black, 2 cycles of Pt-black, and 3 cycles of Pt-black. Figure 3A illustrates the bare gold pillar array along with the different Pt-black surface modifications. As the number of deposition cycles increased, the electrode became more black and powdery. As reported earlier, this over growth of platinum black disturbed the uniformity and adhesion of the platinum black particles [24 ]. We found (Figure 3B) that one deposition cycle gave the highest signal improvement; a 17 fold increase over an array that did not have any Pt-black (bare array). Two or three deposition cycles had a 7 and 8 times (respectively) signal increase over the bare pillar array, but were lower than the 1 cycle deposition. For subsequent studies, a 1 cycle Pt-black deposition process was utilized. When using with microchip-based flow analysis and standards derived from NO gas, the gold pillar array modified with Pt-black showed an LOD of 5 nM. Clearly, the combination of the array approach and the Pt-black modification result in impressive detection limits for NO.