Optoacoustic imaging (OAI) is an emerging biomedical technique that allows visualization of in-depth tissues by using ultrasonic signals generated by short laser pulses. In this work, the authors combine the optical power of several pulsed high-power diode lasers (HPDLs) at 870 nm and 905 nm to a 7-to-1 675-mum fiber bundle to generate optoacoustic (OA) signals from different mixtures of two gold nanorods solutions with absorbance peak at ~860 nm and ~900 nm, respectively. The pulses produced to generate OA signals are alternated between the two wavelengths by a microcontroller circuit with fast switching (0.5 ms). From the amplitude of the OA signals, the concentrations of the nanoparticles solutions are easily estimated with high accuracy using a fluence model. The results achieved with the proposed system show very good agreement between the concentrations of gold nanorods estimated from measurements and the expected values.