A glass slide with ink drawn on the surface using a black Sharpie brand marker was used for all testing purposes. The black ink had strong absorption through the UV and visible wavelengths and could be accurately positioned in relation to the laser and transducer. Reverse osmosis (RO) water was used for all measurements. Water containing the acridine orange dye (Sigma-Aldrich, USA) and custom-made gold nanoparticles were used for spray systems. Acridine orange was selected as it had strong absorption peaks in the UV and visible wavelengths at 260 nm and 475 nm (Fig. 3C), and the 260 nm absorption peak closely matches the absorption peak of DNA/RNA. Acridine orange is a fluorescent dye with a quantum yield of 0.2 [23 (link)]; while some energy would be lost due to fluorescence, the low quantum yield ensures that most of the energy is absorbed and works well in photoacoustic imaging [24 (link)]. Acridine orange was prepared at three molar concentrations in 10× increments: 3.8 mM (pH 3.8), 0.38 mM (pH 4.3), and 0.038 mM (pH 4.9) using RO water. The gold nanoparticles were synthesized by citrate reduction of chloroauric acid in water [25 (link)]. 250 ml of 0.01 % w/v HAuCl4 solution (gold(III) chloride trihydrate, Sigma-Aldrich 520,918, ≥ 99.9 % trace metal basis) and 5 ml of 1 % w/v sodium citrate solution (sodium citrate dihydrate (Fisher Scientific BP327, 99 %)) were prepared as precursors for nanoparticle synthesis. In total, 100 ml of auric solution was heated to 90 °C; then, 500 µl of 1 % sodium citrate solution was added to the stirred boiling solution. The stirring continued for another 30 min to achieve an 80 nm nanoparticle diameter, confirmed using a Zetasizer Ultra (Malvern, UK) at a pH 9.8 (Fig. 3B). The nanoparticles were then diluted to concentrations of 8 pM, 16 pM, and 32 pM in RO water. The absorption spectrum for the three nanoparticle concentrations is shown in Fig. 3C. Equipment used to prepare the samples includes a ZSA Zeta Series Analytical Balance (ZSA 80, 0.1 mg, Scientech, USA), Orion Star A111 pH meter (Thermo Scientific, USA), Nanodrop 2000c spectrophotometer (Thermo Scientific, USA), UV-3600 spectrophotometer (Shimadzu, Japan), Elite Adjustable-Volume Pipettes (Fisherbrand, USA), Milli-Q water (Q-POD), and calibrated glassware (PYREX, Germany).