Molecularly Imprinted Polymers for Recognition of Engineered Nanoparticles
Authors: Marjanovic Monika , Lieberzeit Peter A. 
Published: July 2021

Introduction

Engineered nanoparticles (ENPs) have found widespread use in modern technology, medicine and daily life. Ag-NPs for example are used as antimicrobials in several medical and cosmetic products, whereas Au-NPs have attracted great interest in medical applications, such as drug delivery systems. However, ENPs can potentially cause adverse toxic effects due to their small size and high surface to volume ratio and their increased utilization in different medical and consumer products leads to an increased exposure to humans. While a range of state-of-the-art technologies, such as ICP-MS, is available for the detection of ENPs, they usually require thorough sample preparation, highly trained personnel and expensive equipment. [1]
To circumvent these limitations, the use of artificial recognition materials, as for instance molecularly imprinted polymers (MIPs), is a promising alternative. MIPs are synthesized via (radical) polymerization in the presence of template molecules. After template removal, cavities, which are complementary to the template in size, shape and chemical functionality, form in the material. These can rebind target molecules with high sensitivity and selectivity. [2] MIPs can be used as receptors for quartz-crystal microbalances (QCMs), which are mass-sensitive transducers that measure a change in mass via a change in resonant frequency of the oscillating quartz. [3]
In this work, we report on developing an easy-to-use and low-cost MIP-based QCM sensor for the detection of ENPs in aqueous matrices.

Method

Stamps with immobilized metal-based NPs were used to imprint template material on poly(styrene-co-EGDMA) layers. Particle distribution and shape of NPs, immobilized on PDMS stamps, were analyzed using atomic force microscopy (AFM). MIPs were prepared by pressing NP-stamps onto poly(styrene-co-EGDMA) thin films. After polymerization overnight, stamps were removed from the polymers and MIPs were characterized via AFM and QCM. For QCM measurements, we utilized an in-house dual-electrode system. In this case, one of the two electrode pairs served as reference and was coated with the non-imprinted polymer (NIP) to compensate for unspecific binding and external effects, such as temperature changes.

Results and Discussion

Two different substrates were tested for fabrication of NP stamps: glass and PDMS. In both cases, APTES was used as linker for NP immobilization and let to desired cavity formation on MIPs as assed via AFM. However, removal of glass stamps was challenging, as a result of the strong interaction between glass NP stamp and the polymer, and required applying rather strong force to remove NP stamps. PDMS led to better outcomes and was easily removable from MIPs, which is especially useful for QCMs due to their fragility. QCM measurements of poly(styrene-co-EGDMA) MIPs, imprinted with Ag-NPs (75 nm, PVP-stabilized), led to high sensor responses of up to 1 kHz, whereas no binding was observed on the NIP. QCMs were analyzed via AFM before and after frequency measurements (figures 1a-d). As shown in figure 1a, cavities matching the template in size and shape were distributed evenly across the MIP surface, whereas the NIP did not reveal any binding sites (1c). After QCM measurement and exposure of MIPs and NIPs to the imprinted Ag-NPs, re-binding occurred exclusively on the MIP as visible in figures 1b and 1d, confirming successful NP imprinting.
First selectivity studies of poly(styrene-co-EGDMA) MIPs seem promising: exposure to smaller Ag-NPs (50 nm) resulted in no re-incorporation of particles on sensing layers. However, measurement of Au-NPs on Ag-NP MIPs, imprinted with particles of the same size and stabilizer shell as the Au-NPs, led to re-binding on the MIP. Those results were not surprising since both NPs had the same size, shape and surface chemistry, and used Au-NPs fitted perfectly into the Ag-NP cavities. However, there is still room for improvement regarding batch-to-batch reproducibility. Further characterization of these sensing materials is necessary and still ongoing.

Conclusion and Outlook

Imprinting of thin polymer-films using metal-based ENPs is a novel concept with considerable potential in the field of sensor technology. Poly(styrene-co-EGDMA) Ag-NP MIPs were prepared by surface imprinting and characterized using AFM, revealing corresponding NP cavities on the polymer surface and thus confirming successful imprinting of NPs. Moreover, QCM measurements of MIPs led to sensor responses with high frequency shifts upon exposure to the imprinted particles with no cross-selectivity to smaller NPs. In the near future, selectivity studies will be completed in order to assess binding responses as a function of NP size, stabilizer and core material.
Overall, this work demonstrates the potential use of MIPs for analysis of NPs in complex matrices, such as food, cosmetic or medical products. However, since biding occurred in an irreversible manner, MIPs would probably be more suitable as filtration materials rather than sensing layers, unless a way is found in order to remove NPs from the cavities again.

Acknowledgements

This work has been funded by the Austrian Science Fund FWF, project number I 3568-N28, which we gratefully acknowledge.

References

[1] A. M. Schrand, M. F. Rahman, S. M. Hussain, J. J. Schlager, D. A. Smith, A. F. Syed, Metal-based nanoparticles and their toxicity assessment, WIREs Nanomedicine Nanobiotechnology. 2 (5), 2010, 544-568. doi: 10.1002/wnan.103.
[2] P. A. Lieberzeit, J. Wackerlig, Molecularly imprinted polymer nanoparticles in chemical sensing – Synthesis, characteriaztion and application, Sensors and Actuators B. 207, 2015, 144-157. doi: 10.1016/j.snb.2014.09.094.
[3] R. P. Buck, E. Lindner, W. Kutner, G. Inzelt, Piezoelectric chemical sensors (IUPAC Technical Report), Pure and Applied Chemistry. 76 (6), 2004, 1139-1160. doi: 10.1351/pac200476061139.

Figure 1