Synthesis and Characterization of Au@MnO<sub>2</sub> Nanoparticles as Plasmon Enhanced Spectroscopy Substrates

The plasmonic properties of Au nanoparticles (AuNP), which allow the observation of enhanced spectroscopic effects, are strongly affected by the aggregation and precipitation caused by the strong interactions between nanoparticles. To avoid AuNP aggregation and precipitation, the present study proposes coating with MnO₂, forming AuNP@MnO₂ core-shell structures. The MnO₂ layers presented 1-10 nm thickness so that highly surface-enhanced fluorescence was obtained with maximum intensity given by 5 nm thick MnO₂. The decrease in Raman intensity could be controlled, despite the inherent reduction in surface-enhanced Raman scattering (SERS) intensity with increasing adsorbate-surface distance. The decrease in Raman intensity was compensated by increasing AuNP stability caused by the MnO₂ shell.

Keywords:
SHINERS; SHINEF; manganese oxide; IR-820; Au nanoparticles

Authorship

Gabriela P. Oliveira

data curation

investigation

methodology

writing-original draft

Laboratório de Nanestruturas Plasmônicas, Núcleo de Espectroscopia e Estrutura Molecular, Centro de Estudos de Materiais, Departamento de Química, Universidade Federal de Juiz de Fora, R. José Lourenço Kelmer s/n, Campus Universitário, 36036-900 Juiz de Fora-MG, BrazilUniversidade Federal de Juiz de ForaBrazilJuiz de Fora, MG, BrazilLaboratório de Nanestruturas Plasmônicas, Núcleo de Espectroscopia e Estrutura Molecular, Centro de Estudos de Materiais, Departamento de Química, Universidade Federal de Juiz de Fora, R. José Lourenço Kelmer s/n, Campus Universitário, 36036-900 Juiz de Fora-MG, Brazil

https://orcid.org/0000-0001-8145-7712

Tatiana B. V. Neves

data curation

methodology

writing-review and editing

Linus P. F. Peixoto

formal analysis

methodology

writing-review and editing

Sandra M. Landi

formal analysis

methodology

writing-review and editing

Divisão de Metrologia de Materiais, Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMETRO), Av. Nossa Senhora das Graças, 50, Prédio 3, 25250-020 Duque de Caxias-RJ, BrazilInstituto Nacional de Metrologia, Qualidade e Tecnologia (INMETRO)BrazilDuque de Caxias, RJ, BrazilDivisão de Metrologia de Materiais, Instituto Nacional de Metrologia, Qualidade e Tecnologia (INMETRO), Av. Nossa Senhora das Graças, 50, Prédio 3, 25250-020 Duque de Caxias-RJ, Brazil

Bráulio S. Archanjo

formal analysis

methodology

writing-review and editing

Gustavo F. S. Andrade ^**e-mail: gustavo.andrade@ufjf.br

conceptualization

formal analysis

funding acquisition

supervision

writing-review and editing

https://orcid.org/0000-0003-0718-9400

*e-mail: gustavo.andrade@ufjf.br

Editor handled this article: Jaísa Fernandes Soares

SCIMAGO INSTITUTIONS RANKINGS

Figures | Tables | Formulas

Figures (4)
Tables (3)
Formulas (1)

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Figure 1
UV-Vis spectra of Au@MnO₂ core-shell nanoparticles for different shell thicknesses. All spectra have been normalized for the extinction at the top of the LSPR band.

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Figure 2
(a) and (b) different magnifications of TEM Au@MnO₂ (using Frens²⁹29 Frens, G.; Nat. Phys. Sci. 1973, 241, 20. [Crossref]
Crossref... 50 nm AuNP) with 2.5 nm thickness. (c) Digital zoom of (b) with Au interatomic distance measurement. (d) EDS spectrum confirms the core-shell-nanoparticle chemical composition. (e) Composite HAADF images of Au@MnO₂ (using Frens²⁹29 Frens, G.; Nat. Phys. Sci. 1973, 241, 20. [Crossref]
Crossref... 16 nm AuNP) with 2.5 nm thickness and the chemical image for (f) Au and (g) Mn.

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Figure 3
Fluorescence spectrum of IR-820 1.0 × 10^-5 mol L^-1 and SEF/SHINEF spectra of IR-820 in the presence of AuNPs and Au@MnO₂ with 2.5, 5.0, and 10 nm nominal MnO₂ thicknesses (λ₀ = 785 nm).

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Figure 4
Raman spectra and SHINERS on Au@MnO₂ at different thicknesses, as shown in the figure, of IR-820 1.0 × 10^-5 mol L^-1 in solution.

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Table 1
Volumes of 10 mmol L^-1 KMnO₄ and Na₂C₂O₄ solutions used for achieving different nominal thicknesses of MnO₂ over the Au nanoparticles (16 nm)

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Table 2
SHINEF AEF for four different bands of the IR-820 spectra on Au@MnO₂ as a function of the nominal thickness of the manganese oxide overlayer, calculated according to equation 1, for the spectra excited at 785 nm

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Table 3
SHINERS AEF for four different bands of the IR-820 spectra on Au@MnO₂ as a function of the thickness of the manganese oxide overlayer, calculated according to equation 1, for the spectra excited at 633 nm

Thumbnail

(1)

Figure 1 UV-Vis spectra of Au@MnO₂ core-shell nanoparticles for different shell thicknesses. All spectra have been normalized for the extinction at the top of the LSPR band.

Figure 2 (a) and (b) different magnifications of TEM Au@MnO₂ (using Frens²⁹29 Frens, G.; Nat. Phys. Sci. 1973, 241, 20. [Crossref]
Crossref... 50 nm AuNP) with 2.5 nm thickness. (c) Digital zoom of (b) with Au interatomic distance measurement. (d) EDS spectrum confirms the core-shell-nanoparticle chemical composition. (e) Composite HAADF images of Au@MnO₂ (using Frens²⁹29 Frens, G.; Nat. Phys. Sci. 1973, 241, 20. [Crossref]
Crossref... 16 nm AuNP) with 2.5 nm thickness and the chemical image for (f) Au and (g) Mn.

Figure 3 Fluorescence spectrum of IR-820 1.0 × 10^-5 mol L^-1 and SEF/SHINEF spectra of IR-820 in the presence of AuNPs and Au@MnO₂ with 2.5, 5.0, and 10 nm nominal MnO₂ thicknesses (λ₀ = 785 nm).

Figure 4 Raman spectra and SHINERS on Au@MnO₂ at different thicknesses, as shown in the figure, of IR-820 1.0 × 10^-5 mol L^-1 in solution.

Table 1 Volumes of 10 mmol L^-1 KMnO₄ and Na₂C₂O₄ solutions used for achieving different nominal thicknesses of MnO₂ over the Au nanoparticles (16 nm)

Nominal MnO₂ overlayer thickness / nm	0.8	1.2	2.5	3.5	5.0	10
Volume of 10.0 mmol L^-1 KMnO₄ / µL	2.7	43.6	105.0	164.5	270.0	855.0
Volume of 10.0 mmol L^-1 Na₂C₂O₄ / µL	0.54	8.7	21.0	32.9	50.0	171.0

Table 2 SHINEF AEF for four different bands of the IR-820 spectra on Au@MnO₂ as a function of the nominal thickness of the manganese oxide overlayer, calculated according to equation 1, for the spectra excited at 785 nm

SHINEF AEF
AuNP	2.5 nm	5 nm	10 nm
0.085	6.6	11.4	2.5

SHINEF AEF: shell-isolated nanoparticles enhanced fluorescence analytical enhancement factor.

Table 3 SHINERS AEF for four different bands of the IR-820 spectra on Au@MnO₂ as a function of the thickness of the manganese oxide overlayer, calculated according to equation 1, for the spectra excited at 633 nm

υ / cm^-1	SHINERS AEF
υ / cm^-1	1.2 nm	2.5 nm	5 nm	10 nm
1532	86	46	60	49

SHINERS AEF: shell-isolated nanoparticles enhanced Raman spectroscopy analytical enhancement factor.

(1)

A E F = \frac{I_{S E} / C_{S E}}{I_{N E} / C_{N E}}

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