Nanoscale Structural Switching of Plasmonic Nanograin Layers on

Subscriber access provided by UNIVERSITY OF THE SUNSHINE COAST

Article

Nanoscale Structural Switching of Plasmonic Nanograin Layers on Hydrogel Colloidal Monolayers for Highly Sensitive and Dynamic SERS in Water with Areal Signal Reproducibility Ji Eun Song, Hakseong Kim, Sang Wook Lee, and Eun Chul Cho Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01021 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Analytical Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Analytical Chemistry

Revised MS, ac-2017-010216.R2

Nanoscale Structural Switching of Plasmonic Nanograin Layers on Hydrogel Colloidal Monolayers for Highly Sensitive and Dynamic SERS in Water with Areal Signal Reproducibility

Ji Eun Song,†,# Hakseong Kim,‡,# Sang Wook Lee*,§ and Eun Chul Cho*,†

†

Department of Chemical Engineering, Hanyang University, Seoul, 04763, South Korea

‡

Korea Research Institute of Standards and Science (KRISS), Daejeon, 34113, South Korea

§

Department of Physics, Ewha Womans University, Seoul, 03760, South Korea

*E-mail: [email protected]; [email protected].

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Developing substrates that enable both reproducible and highly sensitive Raman detection of trace amounts of molecules in aqueous systems remains a challenge, although these substrates are crucial in biomedicine and environmental sciences. To address this issue, we report spatially uniform plasmonic nano-wrinkles formed by intimate contact between plasmonic nanograins on the surface of colloidal crystal monolayers. The Au or Ag nanograin layers coated on hydrogel colloidal crystal monolayers can reversibly wrinkle and unwrinkle according to changes in the water temperature. The reversible switches are directed by surface structural changes in the colloidal crystal monolayers while the colloids repeat the hydration–dehydration process. The Au and Ag nano-wrinkles are obtained upon hydration, thus enabling the highly reproducible detection of Raman probes in water at the nano- and picomolar levels, respectively, throughout the entire substrate area. Additionally, the reversible switching of the nanostructures in the plasmonic nanograin layers causes reversible dynamic changes in the corresponding Raman signals upon varying the water temperature.


Page 2 of 27

Page 3 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reproducible and highly sensitive Raman detection of trace amounts of molecules is a crucial issue in various fields.1−6 In air, the intensities of Raman signals can be increased by the local enhancement of the laser electric field on plasmonic substrates,7−9 not only by tailoring the geometry of plasmonic nanoparticles1−6,10 but also by tuning intra-particular10 or inter-particular gap distances.11−17 Concurrently, new plasmonic patterns or nanoparticle-based meta-structures have also been suggested to enable the reproducible and sensitive detection of Raman molecules.18−21 These efforts have lowered detection limits to zeptomole levels.17,21 In aqueous systems, surface enhanced Raman scattering (SERS) measurements are also becoming important for the detection of biological molecules or environmentally harmful species.22,23 However, systems should be designed more carefully to display the function of plasmonic nanoparticles in aqueous systems. Some examples include the reduction of inter-particle or nanoparticle–analyte distances due to the evaporation of water24 or thermally induced de-swelling of hydrogel colloids,25−28 inducing aggregation among nanoparticles29−33 or analyte–nanoparticle contact,34 and alignment of gold nanorods at the water–oil interface.35 Although Raman molecules can be detected in 10-12 M solutions, the SERS signals might not be homogenous throughout the sample or might be changeable depending on alignments, which sometimes limit the reproducible analysis of the samples. In particular, solving these issues could be very important for developing chip-scale detection systems. Thus, in an aqueous system, it remains a challenge to find a new strategy for fabricating plasmonic substrates that can reliably enhance electric fields for both reproducible and highly sensitive SERS signals. Recently, responsive hydrogel colloids have been reported to play pivotal roles in modulating the optical36−41 and magnetic42−44 characteristics of inorganic nanoparticles when the colloids are combined with various inorganic nanoparticles. In Ref. 25–27 and 29, responsive hydrogel colloids were also useful in modulating and enhancing the SERS signals. Such modulations were mostly based on stimuli-dependent dimensional changes in the hydrogel colloids. Meanwhile, to uniformly



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

generate inorganic characteristics in two- or three-dimensional spaces, responsive colloids might be arrayed in an orderly manner on the substrates. However, no investigation has been conducted thus far into the effects of (dynamic or reversible) changes in sizes and crystal arrays of hydrogel colloids on the nanoscale structures of colloidal crystal monolayers. Moreover, this investigation might be important to understanding the impact of nanoscale structures of a colloidal crystal monolayer on the functions of inorganic materials in hybrid multi-dimensional structures. Thus, controlling the nanoscale structures of the colloidal crystal monolayers is expected to offer uniform, reproducible, and dynamic manipulation of the characteristics of inorganic nanomaterials throughout the entire surface area. Additionally, stimuli-responsive and dynamic changes in these characteristic could provide a great deal of meaningful information for biological diagnosis because temperature and other signal changes are fundamental biological indicators for many diseases.45 Herein, we present an approach to developing plasmonic nano-wrinkles wherein Au or Ag nanograins layers are in intimate contact on the surface of hydrogel colloidal crystal monolayers in water, providing highly sensitive and reproducible SERS signals indicating molecules in aqueous systems (Scheme 1). We first find that the crystal monolayers, which are based on stimuli-responsive hydrogel colloids, reversibly alter the surface nanostructures or morphologies while during the hydration–dehydration process in response to temperature changes in water. In addition, such variations in the nanoscale surface morphologies cause the reversible switching of nanostructures of plasmonic (Au or Ag) nanograins coated on temperature-sensitive hydrogel colloidal monolayers in water. Specifically, when the hydrogel colloids are hydrated, the plasmonic nanograin layers (1−2 nm thick) are in intimate contact, thus forming dense plasmonic areas at the colloidal boundaries and developing spatially uniform plasmonic nano-wrinkles throughout the entire substrate. In practice, it is possible to reproducibly detect a trace of a Raman molecule, 1,4-benzenedithiol (1,4-BDT) anywhere in the nano-wrinkles in the aqueous system at a minimum concentration of 500 pM. We demonstrate that the plasmonic nano-wrinkle can also sensitively identify other molecules used in


Page 4 of 27

Page 5 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


biological systems or an environmentally harmful molecule. Moreover, the SERS signal intensities vary reversibly according to the temperature change, which is caused by the switching of the nanostructures of the plasmonic nanograin layers, i.e., from wrinkled to unwrinkled and vice versa. The signal sensitivities to temperature changes provided by this substrate are comparable to or even higher than those of previous works performed with other substrate-type structures.46,47

Scheme 1. A schematic illustrating the preparation of Au nano-wrinkles formed on a colloidal crystal monolayer. A hydrogel colloidal crystal monolayer is formed through the assembly of hydrogel colloids, and plasmonic nanograins were coated on the monolayer. Switching of the nanostructures of plasmonic nanograin layers takes place during the hydration–dehydration switching of colloids according to the water temperature. Below the lower critical solution temperature (LCST, Ttr) of the colloids, plasmonic nano-wrinkles are obtained due to the intimate contact between the plasmonic nanograins at the colloidal boundaries, offering reproducible and highly sensitive SERS signals throughout the entire substrate area. Additionally, the SERS signal intensities of the substrates reversibly vary according to temperature changes, providing dynamic SERS.

■ EXPERIMENTAL SECTION Materials. N,N′-methylene bis-(acrylamide) (99%), allylamine (98%), potassium persulfate (>99%), isopropyl alcohol (IPA, 99.5%), 1,4-BDT (99%), sodium phosphate dibasic dodecahydrate (≥99.0%), sodium phosphate monobasic dihydrate (≥99.0%), thiram (97%) and sodium chloride (99.5%) were purchased from Sigma-Aldrich (Yongin, Korea). N-isopropylacrylamide (98%) was purchased from Wako Chemical, Ltd. (Japan). Sodium hydroxide (96%) was purchased from Duksan



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(Korea). Ethanol (HPLC grade) was purchased from Fisher Scientific (USA). 5-((2-(and-3)-S(acetylmercapto)succinoyl)amino) fluorescein (SAMSA) was purchased from Molecular Probes (USA). Hydrochloric acid (35%) was purchased from Junsei Chemical (Japan). All the chemicals were used as received. Synthesis of Poly(N-isopropylacrylamide-co-allylamine) Hydrogel Colloids. In a two-neck 250 mL round-bottom flask, N-isopropylacrylamide (2.0 g) and N,N′-methylene bis(acrylamide) (0.08 g) were dissolved in deionized water (100 mL) and heated to 80 °C. Subsequently, 70 µL of allylamine and next 2 mL of a potassium persulfate aqueous solution (0.025 g/mL) were added to the reactor. The solution was stirred at 80 °C for 2 h for the reaction to proceed. The resultant solution was quickly cooled in an ice bath and stored at 4 °C. The hydrodynamic diameter of the hydrogel colloids was measured using dynamic light scattering (DLS, Nano-ZS90, Malvern Instruments, UK). Coating Au Nanograin Layers onto Colloidal-Crystal Monolayers. The hydrogel colloid aqueous dispersion was centrifuged, re-dispersed in ethanol, and centrifuged again. The obtained hydrogel particles were finally re-dispersed in IPA. To form the hydrogel colloidal crystal monolayer, a Petri dish was filled with deionized water. Then, a monolayer film was formed at the air–water interface by dropping the hydrogel colloids dispersed in IPA onto the water surface. The formed monolayer film was transferred onto the Si wafer (1×1 cm2), which had been dipped in the water, by lifting the substrate horizontally to the water surface. After drying the monolayer under ambient conditions, the wafer was loaded into an ion-beam evaporator (Daeki Hitech Co. Ltd, Korea), and Au or Ag nanograins were deposited at a deposition rate of 0.3 Å/s. SERS Measurements. 1,4-BDT, SAMSA and thiram were employed as the SERS probe molecules. First, 1,4-BDT was bonded to the Au or Ag nanograins deposited on the colloidal crystal monolayer by separately immersing the samples in 5 mL of various 1,4-BDT ethanol solutions (100 pM−5 mM) for 19 h. Then, each sample was taken out, repeatedly washed with fresh ethanol to remove residual 1,4-BDT, and dried under vacuum at room temperature. For SAMSA-bonded SERS


Page 6 of 27

Page 7 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


substrate, SAMSA was bonded to the Ag nanograins deposited on the colloidal crystal monolayer by following a method from Ref. 48. To activate SAMSA, 10 mg of SAMSA was dissolved in 1 mL of a sodium hydroxide (0.1 M) aqueous solution and incubated for 15 min to remove the acetyl group (the thiol protecting group), followed by neutralization with 14 µL of a 6 M hydrochloric acid aqueous solution. Then, the reagent solution was buffered by adding 0.2 mL of sodium phosphate (0.5 M, pH 7.0) and diluted to the desired concentration (1−100 nM) with ethanol. The plasmonic nanograins layer on the colloidal crystal monolayer was immersed in 5 mL of SAMSA ethanol solution for 30 min. Finally, the samples were washed with ethanol three times and then dried at room temperature under vacuum. For thiram-bonded SERS substrate, thiram was bonded to the Au nanograins coated on colloidal crystal monolayer by immersing the substrates in 5 mL of thiram aqueous solution (50 nM−1 µM) for 2 h, followed by washing three times with water and then dried at room temperature. The Raman spectra were obtained using a confocal Raman microscope (WITec Alpha 300, WITec, Germany) with an Ar laser excitation (532 nm, 6.5 mW power). In most experiments, Raman spectra were recorded with a signal integration/acquisition time of 60 s in the 230−3700 cm-1 spectral range. Meanwhile, for Raman mapping (50×50 µm2, 900 points, 1.6 µm step width in the X- and Y-directions), Raman spectra were recorded with a 5 s signal integration time. A 10× objective lens (NA 0.3) was used to focus the laser onto the samples, which were laid in a cuvette-like glass chamber (2.2×2.2 cm2) filled with deionized water. For the temperature-dependent SERS measurement in the water environment, the water temperature was controlled using a miniature hot plate and measured using a thermocouple inside the sample chamber. The water temperature was modulated in the range from 25 °C to 50 °C at intervals of 5 °C, and each temperature was equilibrated for 20 min for reliable environmental conditions.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 27

Characterization. The root-mean-square roughness of the colloidal crystal monolayer and plasmonic nanograins deposited on the monolayer was investigated by conducting a sectional analysis using topography images recorded by atomic force microscopy (AFM, NX10, Park Systems, Korea). The images were obtained in the non-contact mode at a scan rate of 0.5 Hz. Commercial Si cantilevers (PPP-NCHR, NANOSENSORS, Switzerland) with a length of 125 µm were used with a radius of curvature of less than 10 nm and a nominal force constant of 42 N/m at a resonance frequency of 330 kHz. To indirectly estimate the thickness of the nanograins deposited on the monolayer (Figure S3), a bare silicon substrate locally covered with a shadow mask was attached next to the monolayer samples during each step of metal evaporation. By measuring the thickness of the deposited area on the bare silicon substrate through AFM, the thickness of the nanograins on monolayer was estimated. Scanning electron microscopy (SEM, S-4800, HITACHI, Germany) was also used to investigate the morphologies of the monolayer array and the deposited Au or Ag nanograins on the array. For the monolayer and Au/Ag-deposited colloidal crystal monolayers prepared at 25 and 50 °C, the samples were immersed in deionized water at 25 °C and 50 °C, respectively, and completely dried while maintaining their temperature. Enhancement Factor Calculation. The SERS enhancement factor (EF) was calculated for the strongest Raman band of the 1,4-BDT molecules using the following equation:49 EF = (ISERS/NSERS) / (IRaman/NRaman)

(1)

Where ISERS and IRaman represent the integrated intensities of the 1565 cm-1 peak in both SERS and normal Raman spectra, respectively, and NSERS and NRaman denote the number of 1,4-BDT molecules contributing to the SERS and the normal Raman signals, respectively. IRaman and NRaman were estimated from the Raman spectra of 0.1 M 1,4-BDT in a 12 M NaOH aqueous solution. NSERS was calculated based on the full monolayer adsorption of 1,4-BDT molecules on the Au or Ag nanograins


Page 9 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


with a molecular footprint of 0.54 nm2.50 The Raman measurement conditions, such as the laser wavelength and laser power, were all identical.

■ RESULTS AND DISCUSSION Temperature-Dependent Nanoscale Surface Morphologies of Colloidal Crystal Monolayers in Water. First, the changes in the nanoscale surface structures of colloidal crystal monolayers was investigated. The thermo-sensitive hydrogel colloids as building blocks for the colloidal crystal monolayers were synthesized through the co-polymerization of N-isopropylacrylamide, allylamine, and N,N′-methylene bis-(acrylamide) in an aqueous solution containing potassium persulfate as a free-radical initiator.51 The polymer chains in the colloids exhibit lower critical solution temperature (LCST) behavior in water. Specifically, the LCST is 30−40 °C, and around this temperature, the fully hydrated and coiled polymer chains rapidly dehydrate, and their conformation changes to the globular state.52 Accordingly, the colloids reversibly and steeply altered their hydrodynamic diameters around 30−40 °C in deionized water (FigureS1).53 This size decreased from 440 nm at 25 °C to 200 nm at 50 °C. The colloidal crystal monolayers (1×1 cm2) were prepared using the hydrogel colloids according to a previous reports,54 and the method was also described in Experimental section. Briefly, the purified hydrogel colloids were re-dispersed in IPA, and the colloids were further dropped onto the surface of deionized water to form a monolayer film, which was then transferred onto a Si wafer (1×1 cm2). As shown in Figure 1a and b, the monolayer was well packed with a hexagonal symmetry. From the sectional analysis of an AFM image (indicated by lines and arrows; Figure 1b), the depth of the valleys generated between the two colloids was 11.8 nm (Figure 1c). When the monolayer was immersed in water at 25 °C, the hydrogel colloids in the monolayer appeared to be in closer contact with each other (Figure 1d). Before immersion in water (Figure 1a), the size of the colloids in the monolayer was 324 nm, which was smaller than the sizes (440 nm) of those suspended in deionized



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

water at 25 °C. Therefore, when the temperature of the water was below the LCST, the hydrogel colloids in the monolayer tended to expand. However, the size of colloids only increased to 363 nm. These results imply that instead of further expanding, the colloids were compressed by the neighboring six colloids, causing a large contact area between the colloids. Thus, the depth of valleys generated between the two colloids decreased to 8.0 nm (Figure 1e and f) from 11.8 nm before immersion.

Figure 1. Changes in the morphologies of colloidal crystal monolayers with the variation in water temperature. (a,d,g,j) SEM images, (b,e,h,k) AFM images, and (c,f,i,l) height profiles of the colloidal monolayers (a−c) before and (d−l) after immersion in water at (d−f) 25 °C, (g−i) 35 °C, and (j−l) 50 °C. Height profiles were obtained from a sectional analysis of four AFM images.

When the monolayer was immersed in water at 35 °C (in the middle of the LCST range, Figure 1g), the colloids shrank due to dehydration. In addition, the contact area between the colloids decreased, and the sectional analysis of the AFM topography image (Figure 1h and i) demonstrates that the depth of the valley significantly increased to 37.6 nm (from 8.0 nm at 25 °C). When the temperature of the water was further increased to 50 °C (Figure 1j, above the LCST), the colloids appeared slightly separated from each other. Indeed, the sectional analysis in Figure 1k and l


Page 10 of 27

Page 11 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


indicates that the depth of the valleys was further increased to 49.1 nm. The size of the colloids shrank to 308 nm because they were further dehydrated. However, the sizes were larger than those suspended in deionized water (200 nm; see Figure S1). We speculate that the inter-connections between the colloids in the monolayer might restrict the colloids from freely shrinking, as in the case when they are dispersed in water. Notably, the nanoscale morphology of the monolayers reversibly changed according to repeated heating and cooling in water (Figure S2), similar to that of the colloids dispersed in water (Figure S1). This suggests that the surface structure of the colloid crystal monolayers can be manipulated according to temperature.

Figure 2. Fabrication and characterization of Au nano-wrinkles formed on a colloidal crystal monolayer. (a) AFM image, (b) SEM image, and (c) height profiles of the Au nanograin (0.9 nm thick) layer coated on colloidal crystal monolayers. (d,g) Low- and (e,h) high-magnification SEM images showing the morphologies of the Au-nanograin-coated colloidal monolayer dried while being dipped in water at (d,e) 25 °C and (g,h) 50 °C. (f,i) Respective height profiles of the colloidal monolayer coated with Au nanograin layers for (d) and (g).



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Reversible Wrinkling–Unwrinkling Switching Behavior of Plasmonic Nanograin Layers on the Colloidal Crystal Monolayer and the Fabrication of Plasmonic Nano-Wrinkles. We further investigated how such changes in the nanoscale surface morphologies influence the plasmonic nanostructures formed on the colloidal crystal monolayers. The process for producing and controlling plasmonic nanostructures is as follows. First, Au nanograins were deposited on the monolayer surface (as shown in Figure 1a and b) using an electron-beam evaporator without altering the array of the colloids in the monolayer (Figure 2a). A SEM image (Figure 2b) exhibits tiny Au nanograins (3.5 ± 0.9 nm) evenly deposited on the monolayers. The thickness of the deposited Au layer was 0.9 nm, which was indirectly estimated by measuring the thickness of the Au nanograins deposited on a flat silicon wafer (Figure S3). The sectional analysis of the AFM image shown in Figure 2a shows that the depth of the valley created between the two colloids slightly reduced from 11.8 nm to 9.7 nm (Figures 1c and 2c), which also qualitatively confirms the deposition of a thin Au nanograin layer. The deposition of the Au nanograin layers could be optically confirmed by the UV-visible extinction spectrum (Figure S4) of the Au-nanograin-coated monolayer on a glass plate, which exhibited a peak around 570 nm due to the Au nanograin layer. Interestingly, the Au nano-wrinkles formed in the nanograin layer (Figure 2d and e) when the Aucoated colloidal crystal monolayer was immersed in water at 25 °C (i.e., below the LCST). Bright textured regions in the SEM image (Figure 2e) clearly indicate that higher amounts of secondary electrons were produced due to the formation of dense areas of Au nanograins (note: the SEM images were obtained without any additional metal coating). The size changes of the colloids in the Au-coated colloidal monolayer could be confirmed from the depths of the valley between the two hydrogel colloids. This valley depth reduced to 7.8 nm in water at 25 °C (below the LCST, Figure 2f) from 9.7 nm before immersion in water. Therefore, wrinkles formed due to the intimate contact between plasmonic nanograins located at the colloidal boundaries, probably caused by the compression of the six neighboring colloids. Meanwhile, these nano-wrinkles disappeared when the


Page 12 of 27

Page 13 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature of the water was increased to 50 °C, i.e., above the LCST (Figure 2g and h). The valley depth significantly increased to 20.6 nm at 50 °C (Figure 2f and i), implying that the contact area among the colloids decreased due to the contraction of the colloids. Therefore, the plasmonic nanograin layers became unwrinkled at temperatures above the LCST. Based on these changes in the nanoscale surface morphologies of the colloidal crystal monolayers, the formation of Au nanowrinkles is depicted in Scheme 1. The effect of the Au thickness on the structures of the plasmonic layers was also investigated (SEM images in Figure S5). When 2.4 nm of Au was deposited on the colloidal crystals, the Au nanograins appeared to become larger (6.8 ± 1.1 nm), and some of the nanograins were found to be connected. When the Au-deposited colloidal crystal monolayer was immersed in deionized water at 25 °C, no nano-wrinkles appeared in the SEM images. Nevertheless, the topological analysis of the AFM images revealed that the depth of the valley between two particles still switched from 7.6 nm at 25 °C to 14.5 nm at 50 °C (Figure S6). Importantly, we found that the differences in the depths measured at these two temperatures decreased with the increasing Au thickness (Figure S7). Therefore, an optimum thickness of Au is required to clearly develop the Au nano-wrinkles. With over 3 nm of Au on the monolayer, continuous Au regions became wider, and individual Au nanograins were hardly found. Under these circumstances, the water temperature became ineffective for modulating the Au nanostructures on the monolayer (see the temperature-dependent SERS measurements for various Au thickness in Figure S8). When selecting the optimum thickness of Au nanograin layers for SERS application, it is also worth discussing the effect of grain sizes on SERS signals because the Au thickness increment from 0.9 nm to 2.4 nm accompanied the grain size increase from 3.5 ± 0.9 nm to 6.8 ± 1.1 nm. From SEM images (in Figure 2), their grain sizes were not changed before and after the substrates being exposed to deionized water. We compared the SERS signals of Au nanograin layers with 0.9 nm and 2.4 nm thickness on the colloidal crystal monolayers in water at 50 °C (Figure S8) at which Au nano-wrinkle effect was minimized. The



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

SERS signal was slightly higher with the 0.9-nm-thick Au than with the 2.4-nm-thick Au. Nevertheless, within the sample having identical grain size (whether 0.9-nm or 2.4-nm Au thickness), the difference in SERS signal due to temperature change (25 °C vs 50 °C) was much significant. The results imply that SERS signal changes due to the change in the nanostructures of Au nanograin layers would be a more important factor than the size of Au nanograins.

Figure 3. SERS studies with Au and Ag nanograin layers on the colloidal crystal monolayer. (a) Schematic showing the experimental setup for the temperature-dependent SERS measurement in water. A SERS substrate was prepared by tagging 1,4-BDT as SERS probes on the plasmonic nanograin layers on the colloidal crystal monolayer. (b) SERS spectra of 1,4-BDT-tagged Au nanograin layers (0.9 nm thick) on the colloidal monolayer at various water temperatures. (c) SERS spectra collected from 1,4-BDT-tagged Au nano-wrinkles (0.9 nm thick) on the colloidal monolayer at various concentrations of 1,4-BDT in water at 25 °C. (d,e) SEM images showing the nanostructures of Ag nanograin layers (1.5 nm thick) on the colloidal crystal monolayer. The samples immersed in water were dried at (d) 25 °C and (e) 50 °C. (f) SERS signals measured in water at 25 °C with various concentrations of 1,4-BDT-tagged Ag nano-wrinkles (1.5 nm thick) on the colloidal crystal monolayer. For all SERS studies, the signal acquisition/integration time was 60 s.

SERS Studies with the Plasmonic Nano-Wrinkles. One of the attractive features of the present research for SERS application is that Au nano-wrinkles were constructed without disconnection due to the intimate contact between the Au nanograins on the hydrogel colloids during the hydration of


Page 14 of 27

Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the colloids (Figure 2d and e). Thus, we expect highly enhanced electric fields could be uniformly generated throughout the entire substrate surface. This enhancement effect was confirmed by temperature-dependent measurements of SERS signals of 1,4-BDT chemically attached to the nanostructures. As shown in Figure 3a, an apparatus was devised to measure the SERS signal in an aqueous system at various temperatures (see Experimental section). The 0.9 nm thick Au nanograin layer on the colloidal crystal monolayers was tagged with 1,4-BDT (5 mM) in ethanol, and then, the substrate was immersed in deionized water at 25 °C. The SERS signals were measured with the increasing water temperature (Figure 3b). The highest SERS signal was observed at 25 °C, and this signal intensity was 6−7 times higher than the intensity at 50 °C. The SERS EF was estimated to be 1.34×106 at 25 °C and 2.27×105 at 50 °C (see Experimental section). Regarding the effect of the Au thickness on the SERS intensities measured at 25 °C, the colloidal monolayers bearing Au nanowrinkles (created with 0.9 nm thick Au) exhibited the highest intensities (Figure S9), namely, 6.2 (at 1070 cm-1) and 6.6 (at 1565 cm-1) times higher than the intensity of 6 nm thick Au-coated monolayer. Figures 3b and S9 demonstrate that an enhanced field is generated in the Au nano-wrinkles on the colloidal crystal monolayer. We further investigated the detection sensitivity of the Au nano-wrinkles for 1,4-BDT (Figure 3c). A clear SERS signal was observed at 500 nM 1,4-BDT, and traces of the signal were still observed at 1070 and 1565 cm-1 with 50 nM. We deposited Ag on the colloidal crystal monolayers to further increase the SERS sensitivity. From optimization, the nanowrinkles were developed with 1.5 nm of Ag when the Ag-coated colloidal crystal monolayer was immersed in water at 25 °C (Figure 3d). This surface nanostructure was clearly different from those prepared at 50 °C (Figure 3e). Notably, the Ag nano-wrinkles further lowered the detection limit to 500 pM while increasing the EF to 2.11×107 (Figure 3f). Thus, the detection limit approached the highest reported sensitivities (10-11−10-12 M) observed for other Raman molecules using plasmonic nanoparticles in aqueous systems.29,30 We also tested the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sensitivities for another Raman probe (SAMSA), useful as a Raman reporter in biological system, and it was still possible to reproducibly detect the molecule with high sensitivities as low as 5 nM (Figure S10).

Figure 4. (a) SERS signals of 1,4-BDT (concentration: 100 nM) bonded to Au nano-wrinkles (0.9 nm thick) measured at five different positions on the substrate (1×1 cm2). The Au nano-wrinkles were magnified relative to the size of the substrate to visualize the nanostructure. (b) SERS spectra of 1,4-BDT (concentration: 100 nM) bonded to eight different Au nano-wrinkle (0.9 nm thick) substrates. (c) The SERS intensities at 1565 cm-1 from eight different substrates. The present plasmonic nano-wrinkles (with detection limit of Au, 10-8 M; Ag, 10-10 M) were less sensitive than those in previous works (10-11−10-12 M).29,30 However, a clear distinction from their works was that uniform and reproducible (both point-to-point and substrate-to-substrate) SERS signals were obtainable along with a very low detection limit without the concentration of plasmonic colloids into certain areas through magnetic fields. To clearly demonstrate this, we measured the Au nano-wrinkles at five different points, as indicated in Figure 4a (1×1 cm2). We obtained nearly identical SERS signal intensities for all the positions, with the relative standard deviation (RSD) for the 1565 cm-1 peak of 6.78%. We further performed a Raman mapping experiment for the five points at 1565 cm-1. Each of the Raman maps was collected from 900 points with a 1.6 µm step width along both the X-


Page 16 of 27

Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and Y-axes, and thus, a total of 4500 points were obtained. The raw data and Raman mapping images for the 1565 cm-1 band intensity of 1,4-BDT (100 nM) are shown in Figure S11. Note that the SERS signal intensities were lower than those in Figures 3 and 4 because these measurements were performed with a signal acquisition/integration time of 5 s, which is shorter than that of the data shown in Figures 3 and 4 (60 s). More importantly, the RSD value for the 1565 cm-1 band including 4500 points was determined to be 9.25%. Moreover, we also investigated the substrate-to-substrate reproducibility of the SERS signals by obtaining the SERS spectra of 1,4-BDT from eight substrates (Figure 4b, c). The RSD value of the peak intensity at 1565 cm-1 was 9.98%. Since substrates with RSD values below 20% are typically regarded as reproducible SERS substrates,55−57 the present RSD values of less than 10% even at low concentration of 100 nM could demonstrate the substrate’s areal and substrate-to-substrate reproducibility. From a practical perspective, treatment time of target molecules is also an important factor. Since we selected the treatment time of 19 h for the present data with 1,4-BDT, we further acquired SERS spectra with the Au nano-wrinkles which were dipped in 1,4-BDT (100 nM) ethanol solution for 1, 2, 6 and 19 h (Figure S12). Although the signal seemed to be slightly increased as the treatment time increased, the treatment time of 1 or 2 h was also sufficient enough to identify the target molecule. In addition, as in the case of SAMSA (Figure S10), a trace of this molecule is also identifiable with the Ag nano-wrinkles even with 30 min treatment time. To further demonstrate versatility of the present plasmonic nano-wrinkles, we tried to detect thiram, a kind of pesticides broadly used in agriculture as a fungicide but environmentally harmful substance.58 The 0.9 nm thick Au nanograin layer on the colloidal monolayers was treated with thiram (50 nM−1 µM) aqueous solution for 2h, followed by immersion in deionized water at 25 °C. SERS signals due to thiram (at 1383 cm-1) could be



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

sufficiently detected as low as 50 nM (Figure 5). Thus, the chip-sized substrate enables reliable diagnosis and detection of environmentally harmful chemicals.

Figure 5. SERS signals of thiram at 1383 cm-1 measured in water at 25 °C with Au nanowrinkles (0.9 nm in thickness) on the colloidal monolayer. The substrates were treated with various concentrations of thiram aqueous solution for 2 h.

To extend the application, it is worth investigating SERS signals from the present substrate in various aqueous conditions. For examples, body fluids mostly contain sodium phosphate and NaCl in the concentration range of 2−35 mM and 2−140 mM, respectively.59 Thus we measured SERS signals with the Au nano-wrinkles which were tagged with 1,4-BDT in NaH2PO4 or NaCl aqueous solutions (Figure S13). The treatment concentration of 1,4-BDT was 500 nM. Compared with the SERS intensities measured in deionized water, the recorded intensities at 1565 cm-1 and 1070 cm-1 showed a slight change by 3−6% in 35 mM NaH2PO4 or 50 mM NaCl aqueous solution, respectively. Even at highest concentration of NaCl (140 mM), the signal intensities decreased about 10%. Therefore, the results clearly demonstrated that the Au nano-wrinkles could be useful under various aqueous conditions.


Page 18 of 27

Page 19 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Dynamic SERS studies with Au nanograin layers on the colloidal crystal monolayers. (a) Temperature-dependent variation in the SERS intensities at 1565 cm-1 with the 1,4-BDT-tagged Au nanograin layers (0.9 nm thick) coated on the colloidal crystal monolayer during heating (25 to 50 °C) and cooling (50 to 25 °C) processes. (b) Changes in the SERS intensities at 1070 cm-1 and 1565 cm-1 with the 1,4-BDT-tagged Au nanograin layers on the monolayer during repeated heating and cooling processes. The treatment concentration of 1,4-BDT was 5 mM.

Dynamic SERS Studies under the Variation in Water Temperature. Another interesting feature of this study was that the nanostructured plasmonic nanograin layers on the colloidal crystal monolayers also provided suitable substrates for dynamic SERS measurement through the reversible modulation of the SERS intensities with temperature. Figure 6a shows the temperature-dependent SERS intensities of 1,4-BDT covalently bonded to the Au-deposited (0.9 nm thick) colloidal crystal monolayers. The SERS signal at 1565 cm-1 decreased with the increasing temperature (also see Figure 3b). During a cooling cycle, the signal increased again, and more importantly, the signal reversibly changed during repeated heating and cooling cycles (Figure 6b). As illustrated in Figures 1 and 2, these changes in the SERS signal were caused by the reversible switching between wrinkled and unwrinkled plasmonic nanograin layers deposited on the monolayers. The sensitivities of the substrates to



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

temperature changes were evaluated from the ratios of the SERS signal intensities at 25 °C to those at 50 °C (I25 °C/I50 °C). For the 0.9 nm thick Au nanograin layers, the ratios at 1565 cm-1 and 1070 cm-1 were 6.0 (3979/659) and 6.5 (2775/425), respectively. Previously, it was reported that substrate-type SERS sensor systems, consisting of Au nanoparticles tethered by thermosensitive polymer brushes on an Au film or pattern, exhibited ratios of 5.046 and 1.8.47 Thus, the present substrates could display sufficiently high sensitivities in water to indicate temperature changes. In addition, the plasmonic nano-wrinkles and the reversible nanostructure switching of the nanograin layers developed here could potentially be used for reproducible, highly sensitive, and multi-purpose SERS in water. We have also compared the values of I25 °C/I50 °C when using 1,4-BDT (142 g/mol) with those when using SAMSA (521 g/mol) to investigate the effect of sizes of target molecules on the temperature sensitivity (Figure S14). In this case, the treatment concentrations of 1,4-BDT and SAMSA were all 50 µM. The values of I25 °C/I50 °C for 1,4-BDT were 6.2 and 6.4 at 1070 cm-1 and 1565 cm-1, respectively. In addition, the values of I25 °C/I50 °C for SAMSA were also more or less similar, having 6.0 and 6.1 at 1187cm-1 and 1333 cm-1, respectively. Therefore, within the target molecules we investigated, the present substrates could provide high temperature sensitivity, regardless of molecular sizes.

■ CONCLUSIONS We exploited our discovery that the nanoscale surface structures of colloidal crystal monolayers can change in water with variation in temperature for reversible wrinkling–unwrinkling switches made of Au or Ag nanograin layers deposited on the monolayers. From a structural perspective, we expect that controlling the nanoscale surface structure of colloidal crystal monolayers could offer a platform technology for fabricating hybrid nanostructures in which the surface characteristics of functional nanomaterials vary. In addition, the plasmonic nano-wrinkles developed on the monolayer could


Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


provide many practical benefits for detecting hazardous and biological molecules in aqueous systems with both high sensitivity and reliability (from areal and substrate-to-substrate reproducibility). The results of the temperature-dependent reversible SERS signal changes could also provide some meaningful information for biological diagnosis because temperature change is an important biological indicator for many diseases. Taken together, the present studies suggest a method for reproducible and multi-purpose SERS and thus could be very helpful in environmental, chemical, and biological sensing areas. On the other hand, it is worth discussing the other issue with the present systems. The changes in SERS signals upon varying the water temperature might be a hurdle for those who only seek for highly sensitive and reproducible signals or material characteristics. As such case, the water temperature should be properly regulated. Based on our experimental results, the high sensitivity and areal signal reproducibility were obtainable when the water temperature was below 25−27 °C.

■ ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Temperature-dependent hydrodynamic diameters of hydrogel colloids; AFM topologies and height profiles of the colloidal crystal monolayer during the heating– cooling process; Thickness estimation of the Au nanograin layers; An UV-visible extinction spectrum of the Au nanograins (0.9 nm) deposited colloidal monolayer; Au-thicknessdependent morphologies, difference in valley depths (at 25 and 50 °C), and SERS studies of Au-deposited colloidal crystal monolayers; AFM images and height profiles of Au-deposited (2.4 nm) colloidal monolayers; SERS studies for point-to-point reproducibility (Raman mapping); SERS sensitivity of Ag nano-wrinkles (1.5 nm) tagged with SAMSA; SERS studies as a function of treatment time of 1,4-BDT; SERS studies with Au nano-wrinkles for the



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

effect of NaH2PO4 and NaCl on SERS signals; Temperature-dependent SERS studies of the Au nanograins (0.9 nm) tagged with either 1,4-BDT or SAMSA.

■ AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]; [email protected]. Author Contributions #

J.E.S. and H.K. contributed equally to this work.

Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS J.E.S and E.C.C. acknowledges the financial support from NRF grants (2012R1A1A1004697, NRF-2015R1A2A2A01007003, and 2015M3C8A6A06014792) funded by the Ministry of Science, ICT & Future Planning (Korea). H.K. and S.W.L. acknowledges the financial support from the Basic Science Research Program (2015R1A2A2A05050829) and Nano Material Technology Development Program (2012M3A7B4049888). We acknowledge Younan Xia at Georgia Institute of Technology for helpful discussion of the experimental data, Eleanor Campbell at University of Edinburgh for helpful comments and reviews of the manuscript, and Tae Hee Han at Hanyang University for technical supports.

■ REFERENCES (1)

Wang, Y.; Yan, B.; Chen, L. Chem. Rev. 2012, 113, 1391−1428.

(2)

Cao, Y. W. C.; Jin, R. C.; Mirkin, C. A. Science 2002, 297, 1536−1540.


Page 22 of 27

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(3)

Freudiger, C. W.; Min, W.; Saar, B. G.; Lu, S.; Holtom, G. R.; He, C.; Tsai, J. C.; Kang, J. X.; Xie, X. S. Science 2008, 322, 1857−1861.

(4)

Tang, H.; Meng, G.; Huang, Q.; Zhang, Z.; Huang, Z.; Zhu, C. Adv. Funct. Mater. 2012, 22, 218−224.

(5)

Xie, J.; Zhang, Q.; Lee, J. Y.; Wang, D. I. C. ACS Nano 2008, 2, 2473−2480.

(6)

Jin, Y. Adv. Mater. 2012, 24, 5153−5165.

(7)

Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241−250.

(8)

Alvarez-Puebla, R. A.; Agarwal, A.; Manna, P.; Khanal, B. P.; Aldeanueva-Potel, P.; CarbóArgibay, E.; Pazos-Pérez, N.; Vigderman, L.; Zubarev, E. R.; Kotov, N. A.; Liz-Marzán, L. M. Proc. Natl. Acad. Sci. USA 2011, 108, 8157−8161.

(9)

Cheng, L.; Ma, C.; Yang, G.; You, H.; Fang, J. J. J. Mater. Chem. A 2014, 2, 4534−4542.

(10)

Nam, J.-M.; Oh, J.-W.; Lee, H.; Suh, Y. D. Acc. Chem. Res. 2016, 49, 2746−2755.

(11)

Im, H.; Bantz, K. C.; Lindquist, N. C.; Haynes, C. L.; Oh, S. H. Nano Lett. 2010, 10, 2231−2236.

(12)

Cho, W. J.; Kim, Y.; Kim, J. K. ACS Nano 2012, 6, 249−255.

(13)

Wang, H.; Levin, C. S.; Halas, N. J. J. Am. Chem. Soc. 2005, 127, 14992−14993.

(14)

Abu Hatab, N. A.; Oran, J. M.; Sepaniak, M. J. ACS Nano 2008, 2, 377−385.

(15)

Camargo, P. H. C.; Rycenga, M.; Au, L.; Xia, Y. Angew. Chem., Int. Ed. 2009, 48, 2180−2184.

(16)

Li, W.; Camargo, P. H. C.; Lu, X.; Xia, Y. Nano Lett. 2009, 9, 485−490.

(17)

Rodríguez-Lorenzo, L.; Álvarez-Puebla, R. A.; Pastoriza-Santos, I.; Mazzucco, S.; Stéphan, O.; Kociak, M.; Liz-Marzán, L. M.; García de Abajo, F. J. J. Am. Chem. Soc. 2009, 131, 4616−4618.

(18)

Hirai, Y.; Yabu, H.; Matsuo, Y.; Ijiro, K.; Shimomura, M. Chem. Commun. 2010, 46, 2298−2300.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(19)

Ou, Y.; Wang, L.-Y.; Zhu, L.-W.; Wan, L.-S.; Xu, Z.-K. J. Phys. Chem. C 2014, 118, 11478−11484.

(20)

Leng, W.; Vikesland P. J. Anal. Chem. 2013, 85, 1342−1349.

(21)

Ko, H.; Chang, S.; Tsukruk, V. V. ACS Nano 2009, 3, 181–188.

(22)

Wang, Y.; Yan, B.; Chen, L. Chem. Rev. 2012, 113, 1391−1428.

(23)

Álvarez-Puebla, R. A.; Liz-Marzán, L. M. Energy Environ. Sci. 2010, 3, 1011–1017.

(24)

Aldeanueva-Potel, P.; Faoucher, E.; Álvarez-Puebla, R. A.; Liz-Marzán, L. M.; Brust, M. Anal. Chem. 2009, 81, 9233–9238.

(25)

Álvarez-Puebla, R. A.; Contreras-Cáceres, R.; Pastoriza-Santos, I.; Pérez-Juste, J.; LizMarzán, L. M. Angew. Chem. Int. Ed. 2009, 48, 138−143.

(26)

Liu, X.; Wang, X.; Zha, L.; Lin, D.; Yang, J.; Zhou, J.; Zhang, L. J. Mater. Chem. C 2014, 2, 7326−7335.

(27)

Manikas, A. C.; Romeo, G.; Papa, A.; Netti, P. A. Langmuir 2014, 30, 3869−3875.

(28)

Qian, X.; Li, J.; Nie, S. J. Am. Chem. Soc. 2009, 131, 7540–7541.

(29)

Contreras-Cáceres, R.; Abalde-Cela, S.; Guardia-Girós, P.; Fernández-Barbero, A.; PérezJuste, J.; Álvarez-Puebla, R. A.; Liz-Marzán, L. M. Langmuir 2011, 27, 4520−4525.

(30)

Toma, S. H.; Santos, J. J.; Araki, K.; Toma, H. E. Anal. Chim. Acta 2015, 855, 70–75.

(31)

La Porta, A.; Sánchez-Iglesias, A.; Altantzis, T.; Bals, S.; Grzelczak, M.; Liz-Marzán, L. M. Nanoscale 2015, 7, 10377–10381.

(32)

Piorek, B. D.; Lee, S. J.; Santiago, J. G.; Moskovits, M.; Banerjee, S.; Meinhart, C. D. Proc. Natl. Acad. Sci. USA 2007, 104, 18898–18901.

(33)

Tong, L.; Righini, M.; Gonzalez, M. U.; Quidant, R.; Käll, M. Lab Chip 2009, 9, 193–195.

(34)

Quang, L. X.; Lim, C.; Seong, G. H.; Choo, J.; Do, K. J.; Yoo, S.-K. Lab Chip 2008, 8, 2214– 2219.

(35)

Kim, K.; Han, H. S.; Choi, I.; Lee, C.; Hong, S.-G.; Suh, S.-H.; Lee, L. P.; Kang, T. Nat.


Page 24 of 27

Page 25 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Commun. 2013, 4, 2182. (36)

Karg, M.; Pastoriza-Santos, I.; Perez-Juste, J.; Hellweg, T.; Liz-Marzan, L. M. Small 2007, 3, 1222−1229.

(37)

Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 196−201.

(38)

Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175−8179.

(39)

Kuang, M.; Wang, D.; Möhwald, H. Adv. Funct. Mater. 2005, 15, 1611−1616.

(40)

Xu, H.; Xu, J.; Jiang, X.; Zhu, Z.; Rao, J.; Yin, J.; Wu, T.; Liu, H.; Liu, S. Chem. Mater. 2007, 19, 2489−2494.

(41)

Lim, S.; Song, J. E.; La, J. A.; Cho, E. C. Chem. Mater. 2014, 26, 3272−3279.

(42)

Sánchez-Iglesias, A.; Grzelczak, M.; Rodríguez-González, B.; Guardia-Girós, P.; PastorizaSantos, I.; Pérez-Juste, J.; Prato, M.; Liz-Marzán, L. M. ACS Nano 2009, 3, 3184−3190.

(43)

Riedinger, A.; Leal, M. P.; Deka, S. R.; George, C.; Franchini, I. R.; Falqui, A.; Cingolani, R.; Pellegrino, T. Nano lett. 2011, 11, 3136−3141.

(44)

Qian, Z.; Hastings, S. P.; Li, C.; Edward, B.; McGinn, C. K.; Engheta, N.; Fakhraai, Z.; Park, S.-J. ACS Nano 2015, 9, 1263−1270.

(45)

Medzhitov, R.; Schneider, D. S.; Soares, M. P. Science 2012, 335, 936–941.

(46)

Gehan, H.; Fillaud, L.; Chehimi, M. M.; Aubard, J.; Hohenau, A.; Felidj, N.; Mangeney, C. ACS Nano 2010, 4, 6491−6500.

(47)

Nguyen, M.; Kanaev, A.; Sun, X.; Lacaze, E.; Lau-Truong, S.; Lamouri, A.; Aubard, J.; Felidj, N.; Mangeney, C. Langmuir 2015, 31, 12830−12837.

(48)

Chamow, S. M.; Kogan, T. P.; Peers, D. H.; Hastings, R. C.; Byrn, R. A.; Ashkenazi, A. J. Biol. Chem. 1992, 267, 15916−15922.

(49)

Lin, X.-M.; Cui, Y.; Xu, Y.-H.; Ren, B.; Tian, Z.-Q. Anal. Bioanal. Chem. 2009, 394, 1729−1745.

(50)

Camargo, P. H. C.; Cobley, C. B.; Rycenga, M.; Xia, Y. Nanotechnology 2009, 20, 434020.

(51)

Cho, E. C.; Kim, J.-W.; Fernández-Nieves, A.; Weitz, D. A. Nano Lett. 2008, 8, 168−172.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(52)

Cho, E. C.; Lee, J.; Cho, K. Macromolecules 2003, 36, 9929−9934.

(53)

Cho, E. C.; Kim, J.-W.; Hyun, D. C.; Jeong, U.; Weitz, D. A. Langmuir 2010, 26, 3854−3859.

(54)

Moon, G. D.; Lee, T. I.; Kim, B.; Chae, G.-S.; Kim, J.; Kim, S.-H.; Myoung, J.-M.; Jeong, U. ACS Nano 2011, 5, 8600−8612.

(55)

Khlebtsov, B. N.; Khanadeev, V. A.; Panfilova, E. V.; Bratashov, D. N.; Khlebtsov, N. G. ACS Appl. Mater. Interfaces 2015, 7, 6518−6529.

(56)

Daglar, B.; Demirel, G. B.; Khudiyev, T.; Dogan, T.; Tobail, O.; Altuntas, S.; Buyukserin, F.; Bayindir, M. Nanoscale 2014, 6, 12710−12717.

(57)

Kim, H.; Seol, M.-L.; Lee, D.-I.; Lee, J.; Kang, I.-S.; Lee, H.; Kang, T.; Choi, Y.-K.; Kim, B. Nanoscale 2016, 8, 8878−8886.

(58)

Sharma, V. K.; Aulakh, J. S.; Malik, A. K. J. Environ. Monit. 2003, 5, 717−723.

(59)

Hall, J. E. Guyton and Hall Textbook of Medical Physiology, 12th ed.; Sauders: Philadelphia, PA, 2011.


Page 26 of 27

Page 27 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For TOC only


Nanoscale Structural Switching of Plasmonic Nanograin Layers on

Recommend Documents