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Plasmonic nano-bowls for monitoring intra-membrane changes in liposomes, and DNA-based nanocarriers in suspension

Sathi Das, Jean-Claude Tinguely, Sybil Akua Okyerewa Obuobi, Nataša Škalko-Basnet, Kanchan Saxena, Balpreet Singh Ahluwalia, and Dalip Singh Mehta

Open Access Open Access

Abstract

Programmable nanoscale carriers, such as liposomes and DNA, are readily being explored for personalized medicine or disease prediction and diagnostics. The characterization of these nanocarriers is limited and challenging due to their complex chemical composition. Here, we demonstrate the utilization of surface-enhanced Raman spectroscopy (SERS), which provides a unique molecular fingerprint of the analytes while reducing the detection limit. In this paper, we utilize a silver coated nano-bowl shaped polydimethylsiloxane (PDMS) SERS substrate. The utilization of nano-bowl surface topology enabled the passive trapping of particles by reducing mobility, which results in reproducible SERS signal enhancement. The biological nanoparticles’ dwell time in the nano-trap was in the order of minutes, thus allowing SERS spectra to remain in their natural aqueous medium without the need for drying. First, the geometry of the nano-traps was designed considering nanosized bioparticles of 50-150 nm diameter. Further, the systematic investigation of maximum SERS activity was performed using rhodamine 6 G as a probe molecule. The potential of the optimized SERS nano-bowl is shown through distinct spectral features following surface- (polyethylene glycol) and bilayer- (cholesterol) modification of empty liposomes of around 140 nm diameter. Apart from liposomes, the characterization of the highly crosslinked DNA specimens of only 60 nm in diameter was performed. The modification of DNA gel by liposome coating exhibited unique signatures for nitrogenous bases, sugar, and phosphate groups. Further, the unique sensitivity of the proposed SERS substrate displayed distinct spectral signatures for DNA micelles and drug-loaded DNA micelles, carrying valuable information to monitor drug release. In conclusion, the findings of the spectral signatures of a wide range of molecular complexes and chemical morphology of intra-membranes in their natural state highlight the possibilities of using SERS as a sensitive and instantaneous characterization alternative.

© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

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Supplementary Material (1)

NameDescription
Supplement 1       Supplementray Information File

Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Figures (6)
Fig. 1.
Fig. 1. Overview of analysis of nanosized bioparticles used in this study.

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Fig. 2.
Fig. 2. Schematic representation of the fabrication of the Ag coated PDMS nano-bowl SERS substrate. (a) The monolayer bead template is formed at an air-water interface and (b) adsorbed onto a Si substrate. (c) The monolayer is covered with PDMS, then cured, and (d) peeled off generating a nano-bowl geometry. Note that, PS beads template diameter (d0) > Nano-bowl diameter. (e) The nano-bowl is further coated with Ag using a sputtering unit.

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Fig. 3.
Fig. 3. Field emission scanning electron microscopy (FESEM) image of (a-c) uncoated and (d-f) Ag-coated nano-bowl structured PDMS film cured from a PS template of 190 nm, 500 nm, and 1 µm, respectively. Scale bar: 500 nm. The average bowl diameters without any Ag coating were calculated to be 90, 300 and 600 nm, cured from 190, 500 and 1 µm PS beads templates, respectively.

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Fig. 4.
Fig. 4. (a) SEM image of trapped polystyrene beads (250 nm diameter beads) inside the nano-bowl, cured from 1 µm PS template. (b) Shows the maximum displacement of trapped liposome for up to 100 s while the free-floating liposomes were only tracked for up to 4 s due to high Brownian motion. (c) Dynamic track plot of a free-floating liposomes on flat PDMS, (d, e) show trapped and partially trapped liposomes on the nano-bowl shaped PDMS respectively. The plot implies that the trapped liposomes are localized, easy to focus, and while the free and partially trapped liposomes exhibit arbitrary diffusive motion over time. The tracking in (c-e) was only performed in XY plane, the free-floating liposomes had a significant motion along the Z-axis as shown in Supplementary Movie. S.M. 1 and S.M. 2 of Supplement 1.

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Fig. 5.
Fig. 5. SERS spectra of liposomes (a) SPC with and without cholesterol, (b) Corresponding intensity plot where error bar denotes the standard deviation in the signal intensity or 10 independent measurements. (c) SERS spectra of EPE liposome with and without cholesterol, (d) displays the corresponding intensity plot for each prominent peak. The specific peaks at 1555, 1671 cm-1 are occurred for SPC liposome with cholesterols. For EPE liposome with cholesterol, peaks at 1565, 1575, 1617 cm-1 are attributed to cholesterol.

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Fig. 6.
Fig. 6. Comparison of complex DNA specimens: SERS spectra of (a) components of DNA nanogels, nanogel, and modified nanogel membrane with liposome. Equal volumes of the Y-SAF, Y-SAB and L-SAC monomers were then mixed to achieve molar ratios of 4/1/6.5 or 16/4/26 and hybridized at 95 °C for nanogel formation. (b) SERS intensity ratio of Raman features of nanogel and coated nanogel for quantifying signal fluctuations where coating subsequently diminish peaks of DNA (645 cm-1 and 796 cm-1) and enhance characteristics peaks of liposome (1233 cm-1). (c) Characterization of DNA micelles: comparison of micelles with pure DNA, and DNA with cholesterol. Comparison of Drug loaded DNA micelles with individual components of the solution for making drug loaded micelles (highlighted in rectangular box). (d) Corresponding intensity plot where error bar denotes the standard deviation in the signal intensity for 10 independent measurements. The spectrum changes are useful to evaluate controlled drug release.

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Tables (1)

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Table 1. Oligonucleotide sequences of DNA-based nanocarriers

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