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.

Preparation of the traps

Fig. 1: The image shows the preparation of surface patterned topographical templates of PDMS for nanoparticle trapping. A droplet of PDMS solution is put on top of the nanostructured Si master moulds (black substrate). The droplet is spread over the master mould by pressing a glass cover on top. The PDMS follows the lattice structure on the substrate, as can be seen in the hexagonal spread of the PDMS on the second glass slide marked with a red circle.

The preparation of traps consist of two steps: A nanostructured silicon master mould is used to pattern PDMS (compare Fig. 1). In a second step, colloidal silver nanoparticles with different sizes and concentrations are deposited on a variety of the PDMS templates. Fig. 2 (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. 2: (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. 3 (a)). Fig 3 (b) is an image of the meniscus of a nanoparticle droplet during the experiment. For faithful trapping, an Accumulation Zone (AZ) needs to form [3]. Essential in its formation is a temperature variation, starting from above the dew point to a maximum. A small amount of surfactant (Sodium dodecyl sulfate solution) is added to prevent the agglomeration of nanoparticles in the AZ. Another essential parameter to be controlled during the capillary assembly is the contact angle of the droplet because the deposition is carried out by a de-wetting process when the meniscus of the solution moves. To increase the spread of the colloidal droplet on the substrate, an appropriate amount of ethanol is mixed in the particle solution, resulting in a suitable contact angle and ensuring a high assembly yield.

Fig. 3: (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. 4. To ensure a stable colloidal solution, 10 µL of surfactant was added. The deposition started at 10 0C (well above the dew point) and gradually increased to 18 0C in steps of 2 0C. The concentration of the nanoparticles in the 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. 4. 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, trap as well as nanoparticle size plays a vital role during the deposition. Minimization of the free energy in the AZ results in a dense occupation of the entire space by the particles over the volume of the traps. In case the size of the nanoparticles is smaller than the trap size, the deposition of multi-particles in a single trap cavity is possible.

Fig. 4: 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.

To further understand this, a mixture of 50 µL of concentrated Ag   (0.02 mg/mL, 100 nm diameter) nanoparticles diluted with 10 µL ethanol and 5 µL of surfactant was prepared. We performed the deposition on a similar PDMS template at 24 0C. Fig 5 right displays the gradual decrease in particle trapping due to a decrease in the concentration of nanoparticles in the AZ. The uniform colour in the dark field image stands for single-particle trapping inside each trap.

Fig. 5: The dark field microscope images of deposited 100 nm Ag nanoparticles with 0.02 mg/mL concentration. Left image shows the single particle trapping that decreases with time due to loss in nanoparticle concentration in the AZ (right image).

To further understand the role of the trap’s size and shape, a micron size V-shaped structured PDMS substrate was deposited and the experiment repeated with a mixture of 50 µL of concentrated Ag (5 mg/mL, 50 nm diameter) nanoparticles as shown in Fig. 6. Due to the larger size of the cavity, particle trapping is not possible. Instead, a line of particles is seen, attached to the ‘cavity’s boundary forming a cluster instead of single or multi-particle assembly in each trap.

Fig. 6: The dark field microscope images of deposited 50 nm Ag nanoparticles with 5 mg/mL concentration on a large cavity with micron size dimension. Image to the left: beginning of deposition with high concentration in the AZ and to the right: after some time with drop in particle concentration within the AZ.

Summary

During my visit at KTU, I was able to trap particles with two different sizes (diameter 50 nm and 100 nm). Repeating the experiments at the MCI, we prepared suitable traps using structured Si master moulds that we produced via electron-beam lithography (EBL) and etching. A challenge remains the optimization of crucial parameters for CAPA: the concentration of the nanoparticles in the droplet, the temperature during deposition, the speed of X-Y stage 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

  1. Chen, Jiaming, et al. “Application of ordered nanoparticle self-assemblies in surface-enhanced spectroscopy.” Materials Chemistry Frontiers5 (2018): 835-860.
  2. 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.
  3. Virganavičius, D., et al. “Investigation of transient dynamics of capillary assisted particle assembly yield.” Applied Surface Science406 (2017): 136-143.