PMSQ powder, average molecular weight 7,465 g mol−1, was purchased from Wacker Chemie AG, GmbH, Burghausen, Germany. Liquid PFH was provided by F2 Chemicals Ltd., Lea, UK (purity grade, 99.7–100 %; density, 1,710 kg m−1). Ethanol was procured from the Sigma-Aldrich (Poole, UK; purity grade, 99.7–100 %; density, 790 kg m−1).
5, 10, 20, 30, 40 and 50 wt% PMSQ was dissolved in ethanol in a sealed vial for 1,800 s at ambient temperature (23 ± 2 °C), using a magnetic stirrer.
Characterisation of solutions
The standard data sheet of F2 Chemicals Ltd. provided the physical properties of PFH. The polymer solutions were characterised to measure surface tension, viscosity and density using calibrated equipment. A VISCOEASY-L rotational viscometer (Schott GERÄTE GMBH, Germany) and an Ostwald U-tube viscometer were used to measure the viscosity. A tensiometer K9 (Kruss GmbH, Germany, standard Wilhelmy plate method) was used to determine the surface tension. A standard 25-ml density bottle was used to measure the density. All experiments were conducted at the ambient temperature (23 ± 2 °C), and ethanol was utilized as a cleaning and standardising agent prior to characterisation experiments.
Preparation of nanospheres
A transparent V-shaped microfluidic junction (VMJ) device was designed and constructed using polymethylmethacrylate (PMMA) with dimensions of 22 × 27 × 15 mm and was used to prepare the polymer nanospheres. Teflon-fluorinated ethylene polypropylene (TEP) capillaries with internal and external diameters of 100 µm and 1.6 mm, respectively, were used to provide continuous flow of the PMSQ solutions (5–50 wt%) and PFH from high precision pumps (Harvard PHD 4400, Apparatus, Edenbridge, UK) to the VMJ device. A schematic illustration of the preparation of solid polymer nanospheres is depicted in Fig. 1. As shown, the PMSQ solutions and PFH are fed from 10-ml plastic syringes (Becton Dickinson, Oxford, UK) using the high precision pumps and the V-shaped channels of the microfluidic junction (Fig. 1a). All liquids mixed in the centre of the device where the channels of the microfluidic junction meet. Subsequently, formation of droplets occurred. These resultant droplets are then guided down an exit channel (outlet capillary) placed at the bottom, and droplet clusters are collected at the channel exit (Fig. 1b). Upon impact with the water in the collector, the droplet is disrupted and releases the volatile solvent while the polymeric material forms nanospheres (Fig. 1c). Resulting nanospheres were collected in a glass vial filled with distilled water.
Optimization studies were conducted to obtain monodisperse nanospheres by varying the polymer (PMSQ) concentration (5–50 wt%), the flow rate of the PMSQ solutions and of the PFH (in the range 50–300 µl min−1). The flow processes were observed using a Phantom V7 high speed camera (provided by Engineering and Physical Science Research Council of the UK).
Characterisation of nanospheres
Droplets were observed using a Nikon Eclipse ME-600 optical microscope (Nikon Co, Tokyo, Japan) as soon as they were generated. Samples of collected spheres were left to dry for 48 h at the ambient temperature (23 ± 2 °C) in a desiccator. They were then sputter coated for 200 s to apply a thin layer of gold to prepare them for SEM imaging (5 kV). A JEOL JSM 6301 F SEM was used to characterise the size and morphology of the produced nanospheres. 200 nanospheres were studied using image analysis software (ImageJ 1.47n, Wayne Rasband National Institute of Health, USA).
Transmission electron microscopy (TEM, JEOL JEM 1010) was used to characterise the internal structure of the nanospheres. For TEM, the collected nanospheres were suspended in distilled water and placed on a copper grid (provided by Agar Scientific Ltd., Stansted, UK).
Atomic force microscopy (AFM) was used to investigate the surface of the produced nanospheres. The images were obtained by scanning the resulting spheres kept on a mica surface in air under ambient conditions using an AFM (Bruker Multimode 8.0, Santa Barbara, CA, USA; Veeco Nanoscope analysis software Version V 6.14r1) operated using the tapping mode. Dried samples were scanned by Bruker silicon nitride tips with a force constant of 0.12 N m−1 at 1 Hz with a resolution of 512 × 512 pixels for all images. To avoid structural changes of the sample, the tip loading force was minimized.