Prince Gupta visited the Kaunas University of Technology, Lithuania, for training and hand-on experience on capillary force assisted nanoparticle self-assembly (CAPA). This visit was fruitful and provided insight into the experiment in a broad scientific frame. Read more about the principles of this method and its challenges.
The effect of capillary force offers an effective and promising method to characterize particles with great potential in many spectroscopic and sensing tools [1]. It is, therefore, a promising candidate for a reliable test of nanoparticles in a food matrix. The CAPA method allows to heterogeneously deposition colloidal micro- or nanoparticles on well-defined hydrophobic nanostructured traps with a prefabricated geometry.
Setup
The technique consists of two steps: First, a nanostructured silicon master mold is used to pattern PDMS films. In a second step, colloidal silver nanoparticles with different sizes and concentrations are deposited on a variety of the PDMS templates. Fig. 1 (a)-(c) shows the experimental setup at the Mads Clausen Institute. Each figure displays the details of specific components necessary for the faithful deposition of nanoparticles.
Fig. 1: (a) CAPA setup at the MCI, Sønderborg. 1) Digital camera; 2) Microscope eyepiece; 3) Objective lenses; 4) Experimental setup; 5) Temperature controller; 6) Light source controller; 7) Translation stage controller.
(b) Experimental setup. 1) Sample holder with a vacuum slot; 2) Translation stage; 3) Vertical stage; 4) XY stages.
(c) Sample holder. 1) Glass slide; 2) Rotational stage; 3) Thermocouple; 4) Peltier element.
Principle of the CAPA detection scheme
In capillary force driven assembly, the meniscus of an evaporating droplet containing nanoparticles moves over the structured surface consisting of a two-dimensional array of traps. The evaporating volume near the contact line is responsible for a convective flow of solvent and continuously brings the suspended particles into the meniscus [2]. The meniscus moves in the opposite direction of this convective flow (compare the scheme in Fig. 2 (a)). Fig. 2 (b) is an image of the meniscus of a nanoparticle droplet during the experiment.
Fig. 2: (a) The schematic of nanoparticle deposition driven by capillary force (Soft Matter, 2018,14,2978-2995). Image of meniscus (b).
Experimental
A mixture of 50 µL of concentrated Ag (5 mg/mL, 50 nm diameter) nanoparticles diluted with 200 µL DI water and 200 µL ethanol was deposited on a two-dimensional square lattice of PDMS. The lattice constant was 400 nm, and the diameter of the traps was 100 nm, as can be seen in the dark field microscope images in Fig. 3. The concentration of the nanoparticles in the accumulation zone (AZ)decreases due to particle trapping on the structured surface. Hence, a less dense and inhomogeneous particle distribution becomes visible in the bottom left image (red circle) in Fig. 3. Further, a change in contact angle was introduced, resulting in the almost absence of trapped particles as can be seen in the bottom right image. Since capillary force along with the local confinement by the structured surface is responsible for the permanent trapping, the trap size as well as the nanoparticle size plays a vital role during the deposition. Nanoparticles smaller than the trap size, are successfully trapped in the cavity.
Fig. 3: Dark field microscope images of deposited 50 nm Ag nanoparticles with 5mg/mL concentration. Top left image shows the homogeneous and densely packed trapped nanoparticles. After some time, AZ becomes less concentrated due to loss of nanoparticles in traps and hence the less densely packed trapping can be seen in the top right image. The decrease in nanoparticle concentration leads also to inhomogeneously trapped nanoparticles shown in bottom left image. Any change in contact angle and shape of meniscus breaks the trapping and results in no deposition of particles as shown in bottom right image. The red circle should guide the eye for comparison.
Summary
We were able to trap particles with two different sizes (diameter 50 nm and 100 nm). A challenge remains the optimization of crucial parameters for CAPA: the concentration of the nanoparticles in the droplet, the temperature during deposition, and the contact angle of the fluid on the trap’ s surface. In future, we plan to make master moulds of different sizes and shapes and repeat CAPA for single and multi-particle deposition. The characterization of the samples will be done by Dark Field microscopy and Scanning Electron Microscopy.
References
- Chen, Jiaming, et al. “Application of ordered nanoparticle self-assemblies in surface-enhanced spectroscopy.” Materials Chemistry Frontiers5 (2018): 835-860.
- Ni, Songbo, Lucio Isa, and Heiko Wolf. “Capillary assembly as a tool for the heterogeneous integration of micro-and nanoscale objects.” Soft Matter16 (2018): 2978-2995.
- Virganavičius, D., et al. “Investigation of transient dynamics of capillary assisted particle assembly yield.” Applied Surface Science406 (2017): 136-143.