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Observation of molecular diffusion in polyelectrolyte-wrapped SERS nanoprobes.

DeVetter BM, Sivapalan ST, Patel DD, Schulmerich MV, Murphy CJ, Bhargava R - Langmuir (2014)

Bottom Line: LbL wrapping can be performed within a few hours and does not require the use of organic solvents or hazardous silanes.To minimize diffusion and improve the long-term storage stability of our nanoprobes, two additional nanoprobe preparation techniques were performed: thiol coating and cross-linking of the outer polyelectrolyte layer.In both cases, molecular diffusion is significantly diminished.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, ‡Department of Materials Science and Engineering, §Department of Bioengineering, ∥Beckman Institute for Advanced Science and Technology, ⊥Department of Chemistry, and #Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.

ABSTRACT
The popularity of nanotechnology-based sensing technologies has rapidly expanded within the past decade. Surface-enhanced Raman spectroscopy (SERS) is one such technique capable of chemically specific and highly sensitive measurements. The careful preparation of SERS-active nanoprobes is immensely vital for biological applications where nanoprobes are exposed to harsh ionic and protein rich microenvironments. Encapsulation of optical reporter molecules via layer-by-layer (LbL) polyelectrolyte wrapping is an emerging technique that also permits facile modification of surface chemistry and charge. LbL wrapping can be performed within a few hours and does not require the use of organic solvents or hazardous silanes. Nonetheless, the stability of its products requires further characterization and analysis. In this study, Raman-active methylene blue molecules were electrostatically encapsulated within alternating layers of cationic and anionic polyelectrolytes surrounding gold nanospheres. We observed molecular diffusion of methylene blue through polyelectrolyte layers by monitoring the change in SERS intensity over a period of more than 5 weeks. To minimize diffusion and improve the long-term storage stability of our nanoprobes, two additional nanoprobe preparation techniques were performed: thiol coating and cross-linking of the outer polyelectrolyte layer. In both cases, molecular diffusion is significantly diminished.

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Methylene blue molecules were electrostatically encapsulated betweenPAA and PAH layers. Aliphatic amines from the PAH layer were chemicallycross-linked and stored at 4 °C to help prevent diffusion. (a)SERS spectra of glutaraldehyde cross-linked nanoparticles. Day 1,black; day 6, red; day 12, blue; day 17, purple; day 22, green; day31, gray. (b) Quantification of SERS intensity in terms of equivalentspontaneous Raman signal of methylene blue.
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fig4: Methylene blue molecules were electrostatically encapsulated betweenPAA and PAH layers. Aliphatic amines from the PAH layer were chemicallycross-linked and stored at 4 °C to help prevent diffusion. (a)SERS spectra of glutaraldehyde cross-linked nanoparticles. Day 1,black; day 6, red; day 12, blue; day 17, purple; day 22, green; day31, gray. (b) Quantification of SERS intensity in terms of equivalentspontaneous Raman signal of methylene blue.

Mentions: Chemical cross-linking was investigated as a method for minimizingundesirable diffusion of optical reporter molecules (Figure 4). An amine reactive cross-linker (glutaraldehyde)was added in molar excess to a solution consisting of LbL-encapsulatednanostructures (PAA + MB + PAH). Glutaraldehyde cross-linked availablealiphatic amines on the terminal PAH layer. We observed that aftera 2 h incubation period LbL glutaraldehyde nanoparticles were stableand did not show signs of aggregation as measured by electronic absorptionspectroscopy. In Figure 4, we show that cross-linkednanoparticles are significantly less susceptible to diffusion-dominatedsignal loss. Furthermore, cross-linked samples stored at 4 °Cwere more stable than samples stored at room temperature (Figure S5).


Observation of molecular diffusion in polyelectrolyte-wrapped SERS nanoprobes.

DeVetter BM, Sivapalan ST, Patel DD, Schulmerich MV, Murphy CJ, Bhargava R - Langmuir (2014)

Methylene blue molecules were electrostatically encapsulated betweenPAA and PAH layers. Aliphatic amines from the PAH layer were chemicallycross-linked and stored at 4 °C to help prevent diffusion. (a)SERS spectra of glutaraldehyde cross-linked nanoparticles. Day 1,black; day 6, red; day 12, blue; day 17, purple; day 22, green; day31, gray. (b) Quantification of SERS intensity in terms of equivalentspontaneous Raman signal of methylene blue.
© Copyright Policy
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4334275&req=5

fig4: Methylene blue molecules were electrostatically encapsulated betweenPAA and PAH layers. Aliphatic amines from the PAH layer were chemicallycross-linked and stored at 4 °C to help prevent diffusion. (a)SERS spectra of glutaraldehyde cross-linked nanoparticles. Day 1,black; day 6, red; day 12, blue; day 17, purple; day 22, green; day31, gray. (b) Quantification of SERS intensity in terms of equivalentspontaneous Raman signal of methylene blue.
Mentions: Chemical cross-linking was investigated as a method for minimizingundesirable diffusion of optical reporter molecules (Figure 4). An amine reactive cross-linker (glutaraldehyde)was added in molar excess to a solution consisting of LbL-encapsulatednanostructures (PAA + MB + PAH). Glutaraldehyde cross-linked availablealiphatic amines on the terminal PAH layer. We observed that aftera 2 h incubation period LbL glutaraldehyde nanoparticles were stableand did not show signs of aggregation as measured by electronic absorptionspectroscopy. In Figure 4, we show that cross-linkednanoparticles are significantly less susceptible to diffusion-dominatedsignal loss. Furthermore, cross-linked samples stored at 4 °Cwere more stable than samples stored at room temperature (Figure S5).

Bottom Line: LbL wrapping can be performed within a few hours and does not require the use of organic solvents or hazardous silanes.To minimize diffusion and improve the long-term storage stability of our nanoprobes, two additional nanoprobe preparation techniques were performed: thiol coating and cross-linking of the outer polyelectrolyte layer.In both cases, molecular diffusion is significantly diminished.

View Article: PubMed Central - PubMed

Affiliation: Department of Electrical and Computer Engineering, ‡Department of Materials Science and Engineering, §Department of Bioengineering, ∥Beckman Institute for Advanced Science and Technology, ⊥Department of Chemistry, and #Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.

ABSTRACT
The popularity of nanotechnology-based sensing technologies has rapidly expanded within the past decade. Surface-enhanced Raman spectroscopy (SERS) is one such technique capable of chemically specific and highly sensitive measurements. The careful preparation of SERS-active nanoprobes is immensely vital for biological applications where nanoprobes are exposed to harsh ionic and protein rich microenvironments. Encapsulation of optical reporter molecules via layer-by-layer (LbL) polyelectrolyte wrapping is an emerging technique that also permits facile modification of surface chemistry and charge. LbL wrapping can be performed within a few hours and does not require the use of organic solvents or hazardous silanes. Nonetheless, the stability of its products requires further characterization and analysis. In this study, Raman-active methylene blue molecules were electrostatically encapsulated within alternating layers of cationic and anionic polyelectrolytes surrounding gold nanospheres. We observed molecular diffusion of methylene blue through polyelectrolyte layers by monitoring the change in SERS intensity over a period of more than 5 weeks. To minimize diffusion and improve the long-term storage stability of our nanoprobes, two additional nanoprobe preparation techniques were performed: thiol coating and cross-linking of the outer polyelectrolyte layer. In both cases, molecular diffusion is significantly diminished.

Show MeSH