Polymeric nanoparticles are critical to a wide range of emerging applications, including in-vivo drug and gene delivery, amplification of DNA hybridization biosensors, and immunodiagnostics. [1-3] A reliable, reproducible and efficient production method for synthesizing nanoparticles below 200 nm, however, has yet to be devised. [4] The conventional techniques currently employed, which include solvent evaporation/extraction, [5] spray drying, [6] nanoprecipitation, [7] and, emulsion

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... nking, [8] typically require multi-step procedures, the use of a considerable amount of solvent, and often result in a wide distribution of particle sizes. Other non-conventional techniques overcome these problems but have troubling drawbacks, as in high voltages for electrospraying [9], for example. In this work we demonstrate the use of surface acoustic wave (SAW) atomization together with a nonuniform evaporation and nucleation process to give sub-50-nm diameter monodisperse nanoparticles. SAW atomization is a straightforward and energy efficient technique to generate relatively homogeneous particle size distributions that can be carried out on a chip-scale microdevice for portable drug delivery applications or scaled up for industrial production. It employs technology originally developed for the entirely different purpose of signal filtering and multiplexing, [10] and provides direct control over the size of the particles through adjustment of the operating frequency of the ultrasonic vibration. [11, 12] Here the SAW atomization device is a single interdigital transducer (IDT) consisting of 25 pairs of straight electrodes formed of 250 nm aluminum atop a 4 nm titanium layer in a ba- * Drs. Friend, Yeo, and Arifin are with the Micro-. † Dr. Mechler is with the School of Chemistry, Monash University. sic full-width interleave configuration sputterdeposited onto a 127.68 o Y-x-cut lithium niobate (LiNbO 3 or LN, Roditi UK, Ltd., London) single crystal piezoelectric substrate, as schematically depicted in Fig. 1(a) . Absorption gel (α-gel, Geltec Ltd., Yokohama, Japan) was used to suppress the leftward SAW from the IDT. The SAW wavelength λ was chosen to be 440 μm, thus specifying the electrode and interelectrode gaps along the x-axis at 110 μm. The IDT finger width perpendicular to the x-axis is 10 mm with a gap of 1 mm between the electrode ends and opposite bus bar, which has a width of 3 mm. Application of an oscillating electric field matching the designed resonance frequency of 8.611 MHz at a power of 30 W then induces a shallow (3-4λ deep) electroelastic Rayleigh wave [10] along the substrate surface with a displacement of approximately 10 nm, as illustrated in Fig. 1(b) . Using a syringe pump at a flow rate of 24 ml/hr, we maintained a 25 μl droplet of the working fluid on the substrate in the path of the SAW, composed of 1% w/w poly-εcaprolactone (PCL, Sigma-Aldrich Pty. Ltd., Australia) dissolved in acetone; PCL is a biocompatible and biodegradable polymer which is commonly used for in-vivo controlled release drug delivery. [13] Transmission of acoustic energy into the liquid drop then occurs through leaky SAW, shown in Fig. 1(b) , which is the diffraction of the axially-polarized compressional wave component of the SAW into the fluid at the Rayleigh angle, θ R , defined by the ratio of the bulk wave speeds in the substrate and the fluid; for LiNbO 3 and water, the Rayleigh angle is θ R = 23 o . [12] As the leaky SAW propagates in the fluid, capillary waves are induced at the liquid-air interface due to excitation of lower-order vibration modes at the liquid interfacial membrane. With sufficient power input into the drop such that the acoustic stress overwhelms the capillary stress, desta-1-4244-0608-0/07/$20.00