SERS Detection of Methylene Blue and Crystal Violet Using Silver Nanostars ^†

Abstract

In this work, silver nanostars were synthesized using silver nitrate, ascorbic acid and polyvinylpyrrolidone (PVP), where the concentration of PVP varied from 0.1 to 10 mM. The morphology was studied by scanning electron microscopy where it was observed that the diameter decreases as the PVP concentration decreases. By UV–Vis absorption measurements typical nanostar spectra were found. In addition, surface-enhanced Raman spectroscopy (SERS) substrates were fabricated by depositing silver nanostars over copper film to detect methylene blue and crystal violet. This study allowed us to identify nanostars as an excellent nanostructure for the fabrication of ultrasensitive SERS substrates for the detection of persistent organic pollutants.

1. Introduction

Surface-enhanced Raman spectroscopy (SERS) detection with silver nanostars (AgNS) as substrates have been used to detect organic dyes [1,2,3], fungicides [4], pesticides [5], and molecules related to biological analytes [6] due to their better performance with hotspots that enhanced the Raman signal compared to other nanostructures [7,8,9]. Other applications for detection of DNA mutation [10], Levodopa (drugs essential for the medical treatment of Parkinson disease) [11] or a-Fetoprotein (biomarker associated with many cancer types) [12] illustrate the importance of the silver nanostars.

Verma et al. have reported that is possible to obtain the star morphology by chemical reduction of Ag⁺ using silver nitrate (AgNO₃), L-ascorbic acid (L-AA) and polyvinylpyrrolidone (PVP) [2,13]. They investigated the influence of L-AA and PVP concentrations on the nanoparticle morphology using different [AgNO₃]:[L-AA] molar ratios (R1) ranging between 0.04 and 2 and [AgNO₃]:[PVP] molar ratios (R2) ranging between 2.5 and 50. In this work, silver nanostars were synthesized using silver nitrate, ascorbic acid and PVP with a constant molar ratio R1 of 0.05 and varying only the PVP molarity from 0.1 to 10 mM (36 < R2 < 0.36). The behavior of the morphology and size of the nanostars was studied with scanning electron microscopy and UV–Vis absorption. In addition, SERS substrates were fabricated by depositing silver nanostars over copper film to determine which morphology and size offered the lowest detection limit of methylene blue (MB) and crystal violet (CV). This study allowed us to identify nanostars as an excellent nanostructure for the fabrication of ultrasensitive SERS substrates for the detection of persistent organic pollutants.

2. Materials and Methods

2.1. Materials

Silver nitrate (AgNO₃, ≥99%), Polyvinylpyrrolidone (PVP, MW 40000, 99%), methylene blue (MB), and crystal violet (CV) were purchased from Sigma-Aldrich (Waltham, MA, USA). L-ascorbic acid (L-AA, 99.5%) was purchased from Fagalab. Deionized water was used to dissolve all reagents in all experiments. All materials used were of analytical grade and were used as received without any further purification.

2.2. Fabrication of Silver Nanostars

The colloidal suspensions of AgNS were prepared by chemical reduction of AgNO₃ in a single step using L-ascorbic acid (L-AA) as a reducing agent and PVP as a capping agent. A mixture of a 3.6 mM AgNO₃ solution and a 10–0.1 mM PVP solution (different relative concentrations of PVP were studied in this work as listed in

Table 1. Silver nitrate, L-ascorbic acid and PVP concentrations used for AgNS studied in this work (SP = spheric, ST = star).

AgNO3 [mM]	L-AA [mM]	PVP [mM]	R1 [AgNO3]/[L-AA]	R2 [AgNO3]/[PVP]	Size [μm]	Morphology
3.6	71.5	10	0.05	0.36	0.43	SP
3.6	71.5	5	0.05	0.72	0.36	SP
3.6	71.5	1	0.05	3.60	1.21	ST
3.6	71.5	0.8	0.05	4.50	1.07	ST
3.6	71.5	0.6	0.05	6.00	1.00	ST
3.6	71.5	0.4	0.05	9.00	0.62	ST
3.6	71.5	0.1	0.05	36.00	0.39	ST

) was subjected to magnetic stirring. Subsequently, a 71.5 mM L-AA solution was added dropwise to the mixture without stirring. The color of the final suspension changed from colorless to light grey; depending on the concentration of the PVP solution the gray color became lighter.

2.3. Fabrication of Ag NS-Based SERS Substrates

First, copper substrates with a 2 cm × 2 cm area were cleaned with ethanol, hydrochloric acid, acetone, and deionized water. After drying the substrates, they were added to the AgNO₃-PVP mixture at the same time as L-ascorbic acid was added drop-wise, causing the formation of silver nanostars on the surface of the copper substrates. After 18 h, the AgNS-copper substrates were cleaned with deionized water to remove only the excess silver that was not deposited on the substrate. Next, 500 µL of 1 µM CV solution was dropped onto the AgNS substrates that were then allowed to dry under ambient conditions for one hour. Finally, deionized water was again used to remove excess analyte.

2.4. Characterization

Surface morphology of AgNS samples was investigated with a JEOL JSM 7600F field-emission scanning electron microcope (SEM), the absorbance spectra were obtained with a UV-Vis spectrometer (Thermo Scientific, Genesys 50) and SERS measurements were evaluated using a confocal Raman microscope (Thermo Scientific, DRX) equipped with a laser with 532-nm wavelength and 10-mW power. The SERS measurement was carried out using a laser power of 10 mW and 3 s of collection time.

3. Results and Discussion

Figure 1 shows the absorbance spectra obtained for the AgNS solutions 18 hours after starting the reaction, where a wide peak centered at 420 nm is observed for higher PVP concentrations, which is characteristic of spheric nanoparticles [9,11,14]. As the PVP concentration decreases the wide peak disappears, and the absorbance spectra are transformed to a shape characteristic of nanostar morphology reported by other authors [4,7,10]. It is possible to see how the absorbance decreases as the PVP concentration decreases to below 1 mM.

To explore the morphology of the AgNS, they were deposited on copper foil and then studied with a SEM. Figure 2 shows the SEM images of AgNS found for 5, 0.8, 0.6 and 0.4 mM PVP concentrations. These images are congruent with the absorbance spectra: at high PVP concentrations (5, 10) spherical nanoparticles are formed and, in contrast, at the lowest concentrations AgNS are obtained. Additionally, we found that the average diameter of the AgNS decreases as the PVP concentration decreases from 1.6 μm (1 mM) to 0.4 μm (0.1 mM); see Table 1.

Considering absorption spectra and SEM images, is possible to establish that at the higher concentrations, PVP is adsorbed at full coverage on the Ag nanoparticles such that smaller Ag spherical NP are formed. When the PVP concentration decreases, there is not enough PVP to saturate the surface of the AgNP such that preferential binding of PVP on Ag(100) facets explains the observed anisotropic growth along Ag(111) facets forming the Ag nanostars [15].

For MB detection, 0.5 mL of an aqueous solution with 10⁻⁶ mM of MB was deposited on the surface of AgNS/Cu-foil substrate and left for 1 h before Raman measurements. Figure 3a shows the SERS detection of MB by Ag nanostars at different PVP concentrations with two main peaks at 1436 cm⁻¹ and 1620 cm⁻¹, respectively, associated with vibration modes ν_asym (C–N) and ν_ring (C–C) [16,17]. In Figure 3b the SERS intensity of the 1620 mode as a function of PVP concentration is plotted. A clear tendency is observed where the best detection of MB is with Ag nanostars synthesized at low PVP values.

For CV detection, 0.5 mL of an aqueous solution with 10⁻⁶ mM of CV was deposited on the surface of AgNS/Cu-foil substrate and left for 1 h before Raman measurements. Figure 4a shows the SERS detection of CV by Ag nanostars at different PVP concentrations with easily observed peaks at 418, 439, 726, 804, 915, 1175, 1377, 1585 and 1618 cm⁻¹, associated with vibration modes of CV [18,19]. In Figure 4b the SERS intensity of the 1618 [ν_ring (C–C)] mode as a function of PVP concentration is plotted. The best SERS intensity of CV is with Ag nanostars synthesized at low PVP values, similar to that observed for MB.

The best performance of SERS detection of MB and CV by AgNS synthesized with low PVP concentrations can be associated with the star morphology and the smaller diameters of nanoparticles obtained, suggesting that surface/volume ratio has been maximized.

4. Conclusions

Silver nanostars were synthesized using AgNO₃, L-AA and PVP. The molar ratio R1 (AgNO₃/L-AA) was fixed at 0.05 and the molar ratio R2 (AgNO₃/PVP) ranged between 0.36 and 36 (varying the PVP concentration from 10 to 0.1 mM). Ag spherical nanoparticles were observed at high PVP values, and conversely, Ag nanostars were found at low concentrations. The best performance of SERS detection of methylene blue and crystal violet was observed by AgNS synthesized with low PVP concentrations.