BPS2026 – Stability of KDP microcrystals in biocompatible solvents for nonlinear optical application

Potassium dihydrogen phosphate (KDP) microcrystals are biocompatible nonlinear optical materials with promising potential for label-free multiphoton imaging and therapeutic applications. However, because KDP is a highly ionic salt, it is water-soluble but insoluble in many organic solvents, making stabilization in organic solvents a major challenge. We synthesized KDP microcrystals by antisolvent precipitation using a flow-focusing microfluidic device, where aqueous KDP solution and ethanol were rapidly mixed at a high antisolvent-to-solution ratio. XRD and SEM analyses confirmed a tetragonal (42 m) structure and an average particle diameter of 4.42 μm. The microcrystals were dispersed in ethanol (6 mg/mL) and subsequently diluted in water at different water-to-ethanol ratios (19:1, 9:1, 17:3, and 3:1) to form a water-ethanol co-solvent suspension. Stability was evaluated using dynamic light scattering and zeta-potential measurements. Remarkably, the microcrystals formed a stable colloidal suspension for more than 10 days in aqueous ethanol, despite the high solubility of KDP in water. Second-harmonic scattering (SHS) was investigated using a hyper-Rayleigh scattering setup with a Nd:YAG pulsed laser (1064 nm, 50 mJ/pulse, and 10 ns). A strong and well-defined SHS signal at 532 ± 5 nm was detected from the suspensions, confirming their nonlinear optical properties. These results demonstrate, for the first time, that KDP microcrystals can be stabilized in undersaturated aqueous co-solvent while retaining robust second-harmonic generation. Prolonged colloidal stability combined with efficient second-harmonic generation highlights the unique suitability of potassium dihydrogen phosphate for potential next-generation nanobiophotonics applications, including label-free multiphoton microscopy, controlled drug delivery, and nonlinear photoactivation in photodynamic therapy.