Article pubs.acs.org/JPCC
Controlled Design and Fabrication of SERS−SEF Multifunctional Nanoparticles for Nanoprobe Applications: Morphology-Dependent SERS Phenomena Sehoon Chang,* Shannon L. Eichmann, Ting-Yun S. Huang, Wonjin Yun, and Wei Wang* Aramco Research Center-Boston, Aramco Services Company, 400 Technology Square, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: Dual-mode surface-enhanced Raman scattering (SERS)−surface-enhanced fluorescence (SEF) composite nanoparticles have been developed for possible use as oil reservoir tracers. These composite nanoparticles are composed of metal Ag nanostructured cores, specific dye molecules, and a SiO2 shell coating. Herein, we show that the embedded dye molecules are detectable by both Raman and fluorescence spectroscopies and yield dramatically enhanced detectability due to strong SERS−SEF phenomena with limits of detection (LOD) as low as 1 ppb by fluorescence spectroscopy and 10 ppb by Raman spectroscopy. To determine the optimal structures for signal enhancement for both SERS and SEF, we show how these phenomena are significantly affected by morphologies of the composite nanoparticles. The aggregation status of metal dots and the distance between the metal and dye probe molecules are the crucial factors for enhancement of SERS and SEF signals. Through well-controlled one-pot reactions in microemulsion media, composite nanoparticles with designed morphologies, Ag@SiO2 core−shell structures, or Ag@SiO2/Ag satellite structures have been synthesized, and various dyes have been encoded into these composite nanoparticles. We have demonstrated that the Ag@SiO2/Ag satellite nanoparticles exhibit the highest dye molecule signal enhancement through both SERS and SEF phenomena. Imaging studies on the detection and mobility of these specifically designed nanoparticles in microchannels show their detection within micron-sized pores and at low concentrations. The multifunctional composite nanoparticles presented herein contain different dyes which exhibit different fluorescence emission wavelengths and fingerprinted Raman signals. Thus, these strategically designed nanoparticles provide a possible pathway for future use as barcoded smart reservoir tracers.
1. INTRODUCTION Since the surface-enhanced Raman scattering (SERS) phenomenon was discovered, SERS has been widely studied and considered as one of the most active research topics.1 The phenomenon includes that the weak Raman signal of molecules can be largely enhanced when they are absorbed on roughened or nanostructured metal surfaces. Due to the advances in nanostructure control and synthesis, single-molecule level detection has been achieved by SERS and extensively applied in molecular imaging, as well as chemical, biological, and environmental sensing applications.2−4 As a result, a variety of functionalized plasmonic nanostructures with SERS properties have been demonstrated due to their potential in molecule labeling and tracing applications.5−10 There have been intensive studies to synthesize and control the interparticle junctions to enhance the local electromagnetic field within plasmonic composite nanoparticles for use as ultrasensitive “SERS nanoprobes”. To synthesize efficient composite SERS nanoprobes, various methods such as salt-assisted and silica or polymer support-assisted composite nanoparticles have been developed due to their high densities of reproducible hot spots.11 Specifically, highly uniform, efficient DNA-function© XXXX American Chemical Society
alized core−shell Au nanoparticle SERS nanoprobes with 1 nm nanogap have been demonstrated, and Au nanoparticle templated self-assembly of amphiphilic block copolymer has been used for the control of nanogaps with the sub-10 nm range.12,13 More recently, biopolymers have been used for synthesis of Au core−satellite superstructures as SERS probes.14 In addition to the SERS effect, fluorescence signals can also be enhanced for organic dye molecules by the proximity to noble metal structures with proper spacing between the metal surface and dye molecules, which is a phenomenon called surface-enhanced fluorescence (SEF). The intensity of the fluorescence emission is highly influenced by nonradiative coupling of the fluorophore with localized surface plasmons of metal nanostructures. SEF also increases the absorption of excitation light due to the electromagnetic field enhancement near the metal nanoparticle interface.15,16 Recently, various core−shell dual-function nanoparticles (both SERS and SEF Received: January 21, 2017 Revised: March 17, 2017 Published: March 22, 2017 A
DOI: 10.1021/acs.jpcc.7b00688 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 1. (a) Schematic of synthesis for SERS Ag@SiO2 with Ag satellite composite nanoparticles and TEM images of Ag@SiO2 with Ag satellite composite nanoparticles synthesized with different amounts of reducing agent (b) N2H4, (c) N2H4 with additional N2H4 in cyclohexane, and (d) N2H4 with additional N2H4 in cyclohexane and NaBH4.
acetate (Alfa Aesar) and another one containing 0.2 mL of hydrazine monohydrate (N2H4·H2O, TCI, >98.0%) in microemulsion. Then 0.5 mL of 3-aminopropyltriethoxysilane (APTES, Alfa Aesar, 98%) or 3-aminopropyltrimethoxysilane (APTMS, Gelest) was added as a coupling agent. Amounts of 0.75 mL of ammonium hydroxide (Fisher, certified ACS PLUS, 29.5%) and 2 mL of tetraethylorthosilicate (TEOS, Acros, 98%) were added and stirred overnight for SiO2 encapsulation. The synthesized nanoparticles were collected from microemulsion by adding isopropyl alcohol and then purified by repeated washing with acetone, ethanol, and deionized (DI) water. The final nanoparticle suspension was dispersed in DI water. In situ satellite Ag nanoparticles were synthesized by adding 1 mL of 0.5 M AgNO3 and 0.6 mL of 29.5% ammonia solution into the microemulsion before adding TEOS. Satellite Ag nanoparticles are attached with different amounts of reducing agents: 0.2 mL of N2H4, 0.2 mL of N2H4 with additional 10 μL of N2H4 in 1 mL of cyclohexane, 0.2 mL of N2H4 with additional 10 μL of N2H4 in 1 mL of cyclohexane and 2 mg of NaBH4. The solution was stirred vigorously for 30 min then followed by SiO2 coating. For thick SiO2 encapsulation of composite nanoparticles, we have added twice the amount of TEOS during synthesis. Morphology of the synthesized composite nanoparticles was observed by transmission electron microscopy (TEM). Samples were prepared using a drop-cast technique (with the nanoparticles in ethanol or water) on a carbon lacey film copper grid (200 mesh), and images were taken using a JEOL 2100 thermionic emission TEM under 200 kV. 2.2. Preparation of Ag Nanoparticle SERS Substrate Plates. Hydrophilic and hydrophobic Ag colloids were prepared by the previously developed procedure.23 Glass slides were treated with mercaptopropyl trimethoxysilane (MPTMS) or octyltriethoxysilane (OTES) by refluxing in ethanol solution for 2 h and then drying for 6 h at 60 °C in air.24 A hydrophilic
active) have been demonstrated because of the potential for use in molecular labeling and tracing applications.16−22 Organicmetal-quantum dot hybrid SERS−SEF nanoparticles with additional encoding capacity have been developed for biodetection.8 The preparation of both SERS−SEF active Aucore−Ag-shell nanoparticles by adjusting the distance between fluorophore and metal nanoparticle surfaces using dyeconjugated polyelectrolytes has also been previously demonstrated.16 In our study, we designed and synthesized composite nanoparticle nanoprobes consisting of Ag cores and Ag satellite nanoparticles which exhibit both SERS and SEF phenomena. Through controlled synthesis, the morphologies and interparticle distances could be adjusted and optimized to achieve the highest enhancement for both SERS and SEF signals. Here we focus our discussion on the morphology-dependent SERS phenomena of the SERS−SEF dual function composite nanoparticles, and the detailed study on SEF will be addressed in a separate paper. Furthermore, the barcoding capabilities and the imaging studies in microchannels with detection of SERS− SEF composite nanoparticles within micron-sized pores at low concentrations have been investigated.
2. EXPERIMENTAL SECTION 2.1. Synthesis and Morphology Characterization of Composite Nanoparticles. Ag@SiO2 core−shell nanoparticles were synthesized in water-in-oil microemulsion reaction, which was prepared by a mixture of 27.45 g of Igepal CO-720 (Aldrich, average Mn ∼ 749), 22 mL of 1-hexanol (Alfa Aesar, 99%), and 170 mL of cyclohexane (Fisher, reagent grade). Ag cores were formed by mixing two parts of the microemulsions, one containing 2 mL of 1 M AgNO3 (Fisher, certified ACS reagent, 99.9%), 5 mg of dye fluorescein isothiocyanate (FITC) (Acros, Isomer I, 90% pure), Rhodamine B isothiocyanate (RBITC) (Pfaltz & Bauer, mixture of isomers), or thionin B
DOI: 10.1021/acs.jpcc.7b00688 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 2. (a) Schematic, (b) extinction spectra, and (c) and (d) TEM images of FITC dye embedded SERS Ag@SiO2 with Ag satellite composite nanoparticles.
Raman Mapping. Microfluidic chip testbeds were purchased from Micronit Microtechnologies with two different pore configurations. The pore structure of one chip was a regular cross-hatched array (regular), while the other pores were a connected random structure of circular open zones (irregular). SERS−SEF composite nanoparticles were pumped into the pore space at a concentration of 10−4 wt % with 100 μL/min of flow rate, and then the flow was stopped and allowed to reach equilibrium prior to imaging. For both chips the Raman signal was collected with both a 10× objective (low resolution), to match the fluorescence imaging, and a 50× LWD objective (high resolution) for better resolution. In each case an optical bright field image was taken prior to mapping. Next, using the same instrument parameters as described previously, the Raman spectra were collected at an array of X−Y locations with an accumulation time of 5 s at each X−Y location, and the intensity of the peak at 1320 cm−1 was used as the mapped signal. Depending on the number of X−Y locations to be mapped, a typical scan would take about 3 h.
or hydrophobic Ag nanoparticle layer was deposited onto the slides by soaking the MPTES-treated slide in hydrophilic Ag colloid suspension or the OTES-treated slide in hydrophobic Ag colloid suspension overnight, respectively. Then the hydrophilic and hydrophobic Ag nanoparticle coated slides were used as substrates for dye coating by Langmuir−Blodgett (LB) deposition. FITC or RBITC molecules were first reacted with octadecylamine (OA) in ethanol to form a hydrophobic complex, while the dyes still retain their fluorescence. The hydrophobic dye complex can be spread onto DI water to form a close-packed monolayer at applied surface pressure of ∼20 dyn/cm2 in a LB trough and substantially transferred onto Agcoated slides. A glass slide is coated with hydrophobic silver nanoparticles, and the FITC-OA complex was attached onto the glass with its hydrophobic hydrocarbon tail, which means the fluorophore is not in direct contact with the silver nanoparticles. For the hydrophilic glass on the other hand, the dye polar head is directly in contact with the Ag nanoparticles. 2.3. SERS and Fluorescence Measurement. Raman measurement was conducted with a 532 nm laser in 1.9 mW of power on samples and 10 s collection time using a Horiba spectrometer (LabRAM HR Evolution). Fluorescence spectra of Ag@SiO2/Ag satellite nanoparticles with FITC and RBITC were measured with the corresponding excitation wavelength (495 nm) using a Horiba Nanolog system.
3. RESULTS AND DISCUSSION We synthesized nanocomposite spheres which consist of a Ag metal nanoparticle core, various dye molecules as the SERS reporter, a SiO2 shell for the encapsulation, and satellite Ag nanoparticles for SERS and SEF signal enhancement by hydrolysis of tetraethylorthosilane (TEOS) in microemulaC
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Figure 3. (a) Limit of detection (LOD) of dye-embedded SERS mode optically detectable composite nanoparticles, Raman spectra of various concentrations of composite nanoparticles, (b) Raman spectra of two different thicknesses of SiO2 coating and distances between core and satellite Ag nanoparticles, (c) Raman spectra of the SERS substrates prepared by LB deposition with different distances between Ag nanoparticles and dye molecules, and (d) Raman spectra of the SERS composite nanoparticles coated with various dyes.
tion.25−30 Figure 1(a) shows the schematic of the SERS active nanoparticle design, and transmission electron microscope (TEM) images show the morphology of Ag@SiO2 core−shell nanoparticles with well-decorated additional satellite Ag nanoparticles (Figure 1(b)). From the TEM images, the size of core Ag nanoparticles is approximately 20 nm, and that of the satellite nanoparticles is around 5 nm on average. The composite Ag@SiO2/satellite Ag composite nanoparticles are encoded with specific dye molecules such as FITC, which are detectable by Raman and fluorescence spectroscopies. These composite nanoparticles yield dramatically enhanced detectability due to the SERS−SEF phenomena described above. For satellite Ag nanoparticle attachment on Ag@SiO2 core− shell nanoparticles, we have optimized the amount and the ratio of AgNO3 as the precursor and NaBH4 and N2H4 as the reducing agents during synthesis. As shown in Figure 1(b), (c), and (d), the amount of reducing agent for satellite Ag nanoparticles significantly influences the morphology of resulting composite nanostructures. We find that increasing the amount of reducing agent results in larger Ag satellite nanoparticle aggregates. Adjusting and optimizing the amount of reducing agent (see Experimental Section) provides for uniform, monodispersed Ag@SiO2 core−shell nanoparticles with well-decorated satellite Ag nanoparticles (Figure S1). The specific probing dye molecules, FITC molecules, are selected as a Raman reporter because they can be chemically linked into the SiO2 shell between core and satellite Ag nanoparticles
resulting in a strong SERS signal. Figure 2(b) shows the extinction spectra of FITC dye embedded SERS Ag@SiO2 with Ag satellite composite nanoparticles. The localized surface plasmon resonance (LSPR) wavelength of Ag@SiO2 with Ag satellite composite nanoparticles is red-shifted about 30 nm compared to Ag nanoparticles which is due to the plasmon coupling of core and satellite Ag nanoparticles (Figure 2(b)).14 The presence of the shoulder peak around 499 nm (characteristic FITC adsorption wavelength) indicates that the FITC molecules are in fact embedded in the composite nanoparticle structure.31 Ag@SiO2 with well-decorated Ag satellite composite nanoparticles possess strong SERS properties as indicated by the clean spectral peaks shown in Figure 3(a). To determine the limit of detection (LOD) of Ag@SiO2 with Ag satellite composite nanoparticles as the SERS nanoprobes, we collected Raman spectra from the Ag@SiO2/satellite Ag nanoparticles encoded with FITC dye molecules dispersed in DI water with various concentrations. SERS spectra of the composite nanoparticles in Figure 3(a) clearly show vibrational bands from the C−C bend modes at around 650, 780, and 930 cm−1, a band from C−OH bend mode at 1180 cm−1, and bands from C−C stretch modes at around 1320, 1395, 1480, 1550, and 1620 cm−1 that are specific for FITC.32 Using the signature Raman peak at 1320 cm−1 from the C−C stretch mode of the xanthene ring in FITC, the LOD of Ag@SiO2/satellite Ag nanoparticles was determined as 10 ppb level based on wt % of D
DOI: 10.1021/acs.jpcc.7b00688 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. Raman maps of the regular (a) and irregular (c) pore arrays with additional images showing the pore structure with the Raman map overlain (b), (d).
(100 ppb) demonstrate that Ag nanoparticles with the thin SiO2 result in superior SERS signal (about 7 times) which is attributed to the electromagnetic field effect near the core Ag nanoparticle and satellite nanoparticles having gaps (2 ± 1 nm) compared to Ag nanoparticles with thick SiO2 having gaps (5 ± 3 nm). Furthermore, sub-2 nm interparticle spacing between the satellite Ag nanoparticles of the thin SiO2 shell contributes the stronger SERS effect compared to the satellite Ag nanoparticles on thick SiO2 with 2−5 nm interparticle gaps. As a result, we show that the LOD of SERS Ag@SiO2 composite nanoparticles with a thin SiO2 gives 1 order of magnitude better signal than the Ag@SiO2 composite nanoparticles with the thick SiO2 shell (Figures 3(a) and S3). To further understand the interplay between the effect of dye−particle spacing (between Ag nanoparticles and dye molecules) and the SERS phenomenon, we prepared two different planar Ag nanoparticle-coated SERS substrates coated with dye monolayers attached with and without inert molecules as spacers between the Ag and dye on silica glass slides. The dye molecules FITC or RBITC were first grafted with a hydrophobic hydrocarbon chain through the chemical reaction as shown in Figure S4, and then the modified dye molecules were spread as a monolayer on the water surface and deposited onto Ag-nanoparticle-coated substrates by the LB method. On the hydrophilic substrate, the dye molecules, as polar heads, can directly attach to the Ag surface, while on the hydrophobic substrate, hydrocarbon tails of the modified dye molecules interact directly with the substrate to generate a spacer of ∼2 nm between the Ag and the dye. The insets in Figure 3(c) show schematics of Ag nanoparticles deposited hydrophilic (top) and hydrophobic (bottom) surfaces. The Raman spectra of these substrates are compared in Figure 3(c), in which the Raman intensity from FITC on the hydrophilic substrate is roughly two
nanoparticles in DI water. To estimate the improved LOD of the FITC dye molecules within composite nanoparticles due to SERS and SEF phenomena, we compared the reference curve of fluorescence intensity for FITC dye molecules prepared in DI water (Figure S2). By comparing fluorescence intensities of FITC molecules embedded into our composite nanoparticles in DI water and FITC molecules themselves in DI water, we can estimate the amount of dye embedded in composite nanoparticles. According to the previous literature, the SEF phenomenon provided by noble metal nanoparticles contributes to approximately 1 order of magnitude enhancement in fluorescence intensity,33 and the tangible LOD of encoded dye molecules within Ag@SiO2 nanoparticles can be predicted to be even lower than 10 ppb level. At the same time, LOD of Ag nanoparticles with fluorescence intensity based on wt % of nanoparticles was determined as 1 ppb level. To develop the effective composite nanostructured SERS probes with the best signal enhancement, it is crucial to understand the optimal and effective interparticle spacing between the core particle and dye as well as the optical spacing between the attached satellite nanoparticles.34,35 To study the effect of spacing, we manipulated the morphology during microelmulsion synthesis to produce two different types of SERS composite nanoparticles. In one case (thick coating) the FITC dye molecules are coated on the outside of a composite nanoparticle where the core Ag nanoparticle is covered by a thick SiO2 shell (15 ± 4 nm). In the other case (thin coating), the FITC dye is on the outside of a composite nanoparticle which contains the same core Ag nanoparticles but with a very thin SiO2 shell (about 6 ± 2 nm). TEM images in the inset of Figure 3(b) show the morphology of these two composite nanoparticle morphologies. As shown in Figure 3(b), the Raman spectra of particle dispersions of the same concentration E
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tuned by changing the SiO2 shell thickness and by precisely controlling the separation using hydrophilic/hydrophobic coating by LB deposition. Through this study we gained better understanding of the SERS phenomenon and thus improved our composite nanoparticle design for optimal Ag@SiO2/Ag satellite nanoparticle morphology to give maximized SERS signal. The synthesis and detection procedures were optimized to maximize the SERS signals, and LODs were achieved down to ppb level (wt % of nanoparticles in solution). Through our controlled synthesis of Ag@SiO2/Ag satellite nanoparticles encoded with various dye molecules, each with a different fingerprinted Raman signal, we provide a possible pathway toward SERS−SEF nanoprobe development serving as various barcoded smart reservoir tracers.
times stronger than the intensity on the hydrophobic substrate. This observation clearly demonstrates that when the dye is in direct contact with a rough metal surface the SERS effect is pronounced. To show that this concept also applies to a spherical nanoparticle platform without satellite nanoparticles, we also synthesized two different Ag@SiO2 SERS nanoparticles with dye molecules adsorbed at controlled distances from the core. The first has FITC dye molecules embedded inside composite nanoparticles, and the other has FITC dye molecules coated on the outer shell of the composite nanoparticles. We find that the SERS signal is much stronger when the dye molecules are in contact with core silver nanoparticles as opposed to coated on the outer shell (Figure S5). To demonstrate barcoding capabilities with these composite nanoparticles we produced a variety of composite nanoparticles which encapsulated different dye molecules such as RBITC and thionine in addition to FITC. Each type of Ag@SiO2/Ag satellite composite nanoparticle exhibits different fingerprinted Raman signals which are shown in Figure 3(d). A variety of molecules, such as Alexa Fluor family of fluorescent dyes, are available to encode the composite SERS−SEF NPs with characteristic SERS signals and fluorescence emission in wide spectral range from visible to NIR.36,37 This encoding capability provides a possible pathway toward SERS nanoprobe detection in microfluidic systems. Here we demonstrate microfluidic detection using FITC-embedded Ag@SiO2/Ag satellite nanoparticles in two microfluidic channels with different pore structures. The Raman signal from the embedded FITC was used to map the SERS active composite nanoparticles within the pore space of the regular and irregular channels. First, the optical bright field images were used to confirm that there were no large (>500 nm) aggregates present in the Raman detection region, and separate fluorescence imaging (not shown here) was used to confirm that the particles were mobile in the pore space rather than stuck to the chip surfaces. Figure 4(a) and (c) shows high-resolution Raman maps and white-light image overlays of a single pore position in each chip (Figure 4(b) and (d)) using an accumulation time of 5 s for each spectrum at each X−Y position (see also Figure S6 for low-resolution Raman map). This figure clearly shows the ability to detect low concentrations of diffusing SERS composite nanoparticles confined within small pores. Additionally, these images show that the signal obtained is significantly high to allow for fast detection given that the particles are continuously moving within the capture time. This demonstrates the application of these specially designed particles for imaging and detection within small pores and at low concentration. The stability of the Ag NP@SiO2/Ag satellite nanoparticles for possible reservoir applications has been tested collecting Raman spectra of samples in seawater with salinity of 60 000 ppm. The results were shown in Figure S7, and the overall Raman intensity of Ag NP@SiO2/Ag satellite nanoparticles was slightly decreased but still showed strong SERS signals even after about 1 year of time since they were synthesized.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00688. TEM Images, fluorescence intensity correlation of free dye to nanoparticle embedded dye, Raman spectra for measuring limits of detection (LOD), chemical structures showing the FITC−hydrocarbon grafting reaction pathway, schematics and Raman spectra of core−shell configurations, additional Raman mapping images of microfluidic channels, and Raman spectra of nanoparticles in harsh environments (seawater) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Sehoon Chang: 0000-0002-8007-4354 Wonjin Yun: 0000-0001-8873-1163 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Martin Poitzsch, Jason Cox, and Hooisweng Ow for technical discussion and support. TEM measurement was performed in part at the Center for Nanoscale Systems (CNS), a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF award no. 1541959. CNS is part of Harvard University.
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REFERENCES
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4. CONCLUSIONS We demonstrated highly sensitive Ag@SiO2 with Ag satellite composite nanoparticles as SERS−SEF nanoprobes based on understanding the phenomenological dependence on the distance between Ag nanoparticle surfaces and the probing dye molecules, the interparticle spacing of core and satellite nanoparticles. For this study, the interparticle distance was F
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DOI: 10.1021/acs.jpcc.7b00688 J. Phys. Chem. C XXXX, XXX, XXX−XXX
