The Plasmonic Effect of Gold Nanorods on Charged Molecules: SERRS and SEF effects

Target molecules adsorbed onto metallic nanoparticles can have their Raman and/or fluorescence signals enhanced, leading to the called surface-enhanced [resonance] Raman scattering (SE[R]RS) or surface-enhanced fluorescence (SEF). Here we have applied Au nanorods (AuNRs) coated with a surfactant bilayer leading to a positive surface charge to investigate the role played by these AuNRs in colloidal suspension on SERRS and SEF effects of charged molecules. In the case of the anionic nickel (II) tetrasulfonated phthalocyanine (NiTsPc), besides achieving SERRS with an enhancement factor (EF) of ca. 10⁵, the AuNRs allowed the analytical application of the SERRS effect for the NiTsPc between 8.3x10^-6 and 4.0x10^-5 mol L^-1. The limit of detection of 4.8x10^-7 mol L^-1 (at 752 cm^-1) and 1.3x10^-6 mol L^-1 (at 1338 cm^-1) was found. In the case of the cationic methylene blue, the SEF effect was achieved reaching an EF of ca. 10. Besides, fundamental discussions are carried out considering the results presented here.

Keywords:
gold nanorods; SERRS; SEF; charge effect

Authorship

Tatiana Aparecida Oliveira

Universidade Estadual Paulista (UNESP), Faculdade de Ciências e Tecnologia, Departamento de Física, 19060-900, Presidente Prudente, SP, Brasil.Universidade Estadual PaulistaBrasilPresidente Prudente, SP, BrasilUniversidade Estadual Paulista (UNESP), Faculdade de Ciências e Tecnologia, Departamento de Física, 19060-900, Presidente Prudente, SP, Brasil.

https://orcid.org/0000-0003-1207-3648

Rafael Jesus Gonçalves Rubira

https://orcid.org/0000-0003-0507-8678

Cibely da Silva Martin ^**e-mail: cssmartin@gmail.com

https://orcid.org/0000-0001-5634-525X

Anerise de Barros

Universidade Estadual de Campinas (UNICAMP), Instituto de Química, Laboratório de Materiais Funcionais, 13083-970, Campinas, SP, Brasil.Universidade Estadual de CampinasBrasilSP, BrasilUniversidade Estadual de Campinas (UNICAMP), Instituto de Química, Laboratório de Materiais Funcionais, 13083-970, Campinas, SP, Brasil.

https://orcid.org/0000-0002-1616-2368

Italo Odone Mazali

https://orcid.org/0000-0001-5698-5273

Carlos José Leopoldo Constantino

https://orcid.org/0000-0002-5921-3161

*e-mail: cssmartin@gmail.com

SCIMAGO INSTITUTIONS RANKINGS

Figures | Tables

Figures (6)
Tables (1)

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Figure 1
Molecular structures of (A) nickel(II) tetrasulfonated phthalocyanine (NiTsPc), (B) methylene blue (MB), (C) hexadecyltrimethylammonium bromide (CTAB). (D) AuNR dimension (length and width) distribution obtained from € transmission electron microscopy images (250 AuNRs were counted). Inset in (D): out-of-scale illustration of a AuNR coated with CTAB bilayer (zeta potential of 44 ±1 mV).

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Figure 2
Extinction spectra of AuNR colloid in the absence and presence of (A) NiTsPc (2.2x10^-6, 6.6x10^-6, and 1.6x10^-5 mol L^-1) and (B) MB (2.2x10^-6, 2.2x10^-5, and 5.5x10^-5 mol L^-1). Absorption spectra of both NiTsPc (3.3x10^-5 mol L^-1) and MB (3.3x10^-5 mol L^-1) aqueous solutions are also shown. Insets: molecular structures of (A) NiTsPc and (B) MB.

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Figure 3
Dynamic light scattering (expressed as the percentage of the total scattered light intensity) of AuNR colloid in the absence and presence of NiTsPc (1.6x10^-5 mol L^-1) and MB (2.2x10^-5 mol L^-1). Inset: illustration of rotational and translational diffusion motions of nanorods in the colloid and the end-to-end aggregation in presence of MB.

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Figure 4
(A) SERRS spectrum of NiTsPc (1.6x10^-5 mol L^-1) and RRS spectra of NiTsPc powder and in aqueous solutions (1.0x10^-2 mol L^-1). (B) SEF spectrum of MB (5.5x10^-5 mol L^-1) using AuNR colloid, the fluorescence spectrum of MB in aqueous solutions (1.0x10^-2 mol L^-1), and RRS spectrum of MB powder. The Raman spectrum of neat AuNR colloid is also shown. Laser line at 633 nm.

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Figure 5
SERS spectra of NiTsPc (1.6x10^-5 mol L^-1) and MB (5.5x10^-5 mol L^-1) using AuNRs. Raman spectra of both NiTsPc and MB in an aqueous solution (1.0x10^-2 mol L^-1) are also shown. Spectra applying the baseline correction. Laser line at 785 nm. *Background signal (low intensities) from laser line at 785 nm, which can be observed only when the target molecule (or analyte) shows a poor signal. **Band ascribed to the plastic support.

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Figure 6
SERRS spectra of NiTsPc at different concentrations in AuNR colloid. (A) Increasing the SERRS intensity with increasing the NiTsPc from 8.3x10^-6 to 4.0x10^-5 mol L^-1. Inset: SERRS spectra of NiTsPc from (A) applying the baseline correction. Variation of the SERRS intensity at (B) 752 and (C) 1338 cm^-1 with increasing the NiTsPc concentration. Enhancement factor determined using the normalization from molar concentration ratio for the bands at (D) 752 and (E) 1338 cm^-1. Enhancement factor determined using the normalization by the number of NiTsPc molecules coating the AuNR surface for the bands at (F) 752 and (G) 1338 cm^-1. The SERRS spectra, variation of SERRS intensity, and EF observed for NiTsPc from 8.3x10^-6 to 8.2x10^-5 mol L^-1 are shown in Figure S1 (Supporting Information).

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Table 1
preparation of the colloids (AuNRs+target molecules) used in the UV-vis extinction, DLS, zeta potential, SERS, SERRS, and SEF (before SERRS analytical application) measurements.

Figure 1 Molecular structures of (A) nickel(II) tetrasulfonated phthalocyanine (NiTsPc), (B) methylene blue (MB), (C) hexadecyltrimethylammonium bromide (CTAB). (D) AuNR dimension (length and width) distribution obtained from € transmission electron microscopy images (250 AuNRs were counted). Inset in (D): out-of-scale illustration of a AuNR coated with CTAB bilayer (zeta potential of 44 ±1 mV).

Figure 2 Extinction spectra of AuNR colloid in the absence and presence of (A) NiTsPc (2.2x10^-6, 6.6x10^-6, and 1.6x10^-5 mol L^-1) and (B) MB (2.2x10^-6, 2.2x10^-5, and 5.5x10^-5 mol L^-1). Absorption spectra of both NiTsPc (3.3x10^-5 mol L^-1) and MB (3.3x10^-5 mol L^-1) aqueous solutions are also shown. Insets: molecular structures of (A) NiTsPc and (B) MB.

Figure 3 Dynamic light scattering (expressed as the percentage of the total scattered light intensity) of AuNR colloid in the absence and presence of NiTsPc (1.6x10^-5 mol L^-1) and MB (2.2x10^-5 mol L^-1). Inset: illustration of rotational and translational diffusion motions of nanorods in the colloid and the end-to-end aggregation in presence of MB.

Figure 4 (A) SERRS spectrum of NiTsPc (1.6x10^-5 mol L^-1) and RRS spectra of NiTsPc powder and in aqueous solutions (1.0x10^-2 mol L^-1). (B) SEF spectrum of MB (5.5x10^-5 mol L^-1) using AuNR colloid, the fluorescence spectrum of MB in aqueous solutions (1.0x10^-2 mol L^-1), and RRS spectrum of MB powder. The Raman spectrum of neat AuNR colloid is also shown. Laser line at 633 nm.

Figure 5 SERS spectra of NiTsPc (1.6x10^-5 mol L^-1) and MB (5.5x10^-5 mol L^-1) using AuNRs. Raman spectra of both NiTsPc and MB in an aqueous solution (1.0x10^-2 mol L^-1) are also shown. Spectra applying the baseline correction. Laser line at 785 nm. *Background signal (low intensities) from laser line at 785 nm, which can be observed only when the target molecule (or analyte) shows a poor signal. **Band ascribed to the plastic support.

Figure 6 SERRS spectra of NiTsPc at different concentrations in AuNR colloid. (A) Increasing the SERRS intensity with increasing the NiTsPc from 8.3x10^-6 to 4.0x10^-5 mol L^-1. Inset: SERRS spectra of NiTsPc from (A) applying the baseline correction. Variation of the SERRS intensity at (B) 752 and (C) 1338 cm^-1 with increasing the NiTsPc concentration. Enhancement factor determined using the normalization from molar concentration ratio for the bands at (D) 752 and (E) 1338 cm^-1. Enhancement factor determined using the normalization by the number of NiTsPc molecules coating the AuNR surface for the bands at (F) 752 and (G) 1338 cm^-1. The SERRS spectra, variation of SERRS intensity, and EF observed for NiTsPc from 8.3x10^-6 to 8.2x10^-5 mol L^-1 are shown in Figure S1 (Supporting Information).

Table 1 preparation of the colloids (AuNRs+target molecules) used in the UV-vis extinction, DLS, zeta potential, SERS, SERRS, and SEF (before SERRS analytical application) measurements.

Sample	NiTsPc			MB
	Stock (mol L^-1)	Volume added (μL)	Concentration (mol L^-1)	Stock (mol L^-1)	Volume added (μL)	Concentration (mol L^-1)
1	3.3x10^-4	20	2.2x10^-6	3.3x10^-4	20	2.2x10^-6
2	1.0x10^-3	20	6.6x10^-6	3.3x10^-3	20	2.2x10^-5
3	1.0x10^-3	50	1.6x10^-5	3.3x10^-3	50	5.5x10^-5

Final volume (cuvette) = 3 mL, volume of AuNRs = 250 μL

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[1] *e-mail: cssmartin@gmail.com