Temperature-Controlled Reversible Localized Surface Plasmon

Dec 2, 2013 - The highest sensitivity of this kind of solid sensing platform was obtained by employing chemically synthesized gold nanoprisms for fabr...
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Temperature-Controlled Reversible Localized Surface Plasmon Resonance Response of Polymer-Functionalized Gold Nanoprisms in the Solid State Gayatri K. Joshi,† Kimberly A. Smith,† Merrell A. Johnson,‡ and Rajesh Sardar*,† †

Department of Chemistry and Chemical Biology and Integrated Nanosystems Development Institute, Indiana University−Purdue University Indianapolis, 402 N. Blackford Street, LD 326, Indianapolis, Indiana 46202, United States ‡ Department of Physics, Indiana University−Purdue University Indianapolis, 402 N. Blackford Street, LD 326, Indianapolis, Indiana 46202, United States S Supporting Information *

ABSTRACT: Solid-state temperature responsive localized surface plasmon resonance (LSPR)-based nanosensors were constructed by functionalizing the glass substrate-attached gold nanoprisms with the thermoresponsive polymer poly(allylamine hydrochloride)-co-poly(N-isopropylacrylamide). The robustness of the sensor was enhanced by chemically attaching polymer to the nanoprism surface through an amide coupling reaction versus simple physisorption of polymer onto nanoprism. The highest sensitivity of this kind of solid sensing platform was obtained by employing chemically synthesized gold nanoprisms for fabrication. The surface ligand chemistry significantly influenced the swelling and shrinking transition of the polymer during the temperature variation, which resulted in the alteration of the local dielectric environment of the nanoprisms, modulation of their LSPR properties, and enhancement of sensing efficiency of the nanosensors. Importantly, we have shown for the first time that the dimension of the nanostructure plays an important role in achieving the highest sensitivity for these types of sensors. For example, the edge length of the nanoprisms plays a critical role in the temperature sensitivity of the nanosensors where nanoprisms with ∼28 and ∼40 nm edge lengths displayed ∼10.9 and ∼18.2 nm LSPR dipole peak red-shift, respectively, as the solution temperature increased from 18 to 56 °C. We believe the higher temperature sensitivity for larger edge-length nanoprisms was achieved due to their larger sensing volume. The nanosensors were found to be very stable and displayed high reversibility, which suggests that our temperature-dependent nanosensors can potentially be used as a reversible thermal switch.

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INTRODUCTION Noble metal nanostructures such as gold and silver have shown great potential through a variety of applications in different areas ranging from biological sensors to drug delivery.1−11 Recently, these nanostructures have also been used as active components in design of molecular sensors.12−18 Metal nanostructures display unique localized surface plasmon resonance (LSPR) properties, which are directly influenced by their size and shape2,4,19−29 and most importantly by the change in their local dielectric environment.9,22,24,30−34 Utilizing the latter parameter, various plasmonic-based chemical and biological sensors have been fabricated in which covalent attachment or adsorption of analytes alters the local dielectric environment of the nanostructures and thus their LSPR © 2013 American Chemical Society

properties. Therefore, nanostructures with highly sensitive LSPR spectral responses are required to detect very minute quantities of analytes. In this context, optical properties of nanostructures can be tuned by the stimuli-induced conformational changes of polymers, which in turn alter the local dielectric environment of the nanostructures. Therefore, active plasmonic systems with reversible changes of their optical response can be used for stimulus-responsive sensing. In this article, stimuli-responsive polymer-functionalized gold nanoprisms attached onto silanized glass surface are used for Received: September 16, 2013 Revised: November 4, 2013 Published: December 2, 2013 26228

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Figure 1. Schematic representation of the design of thermoresponsive polymer-functionalized plasmonic nanosensor. The figure is not to scale.

the first time for the design of an ultrasensitive and highly reversible temperature sensor. We have shown that three important parameters control the sensing efficiency of our nanosensor: (1) covalent attachment of polymer onto the nanoprism surface, which enhanced the robustness of the sensors, (2) reactive chemical functional groups present on the nanoprism’s surface that controls the polymer’s phase transition during the temperature change, and (3) edge length of the nanoprisms, which determines their effective sensing volume. To the best of our knowledge, we have observed the highest temperature shifts of this kind of solid-state sensing platform by appropriate selection of the above three parameters. Stimuli-responsive polymers have received substantial interest because of their robust mechanical and chemical properties, which are important in various applications such as thin film surface coating,35 design of smart surfaces and interfaces,36−42 colloidal stabilization,36,37 and microlithographic patterning.38 In addition, thermoresponsive polymers respond to temperature changes by undergoing phase transitions through shrinking and swelling by conformational change in solution. These polymers display a phase separation above their lower critical solution temperatures (LCSTs). In this context, poly(N-isopropylacrylamide) (PNIPAM) is one of the most studied thermoresponsive polymers because of its reversible phase transition property43,44 at a LCST of approximately 32 °C in water. Above the LCST, PNIPAM is present as a hydrophobically collapsed conformational state, whereas below the LCST, it becomes hydrated, which results in an extended conformational state. Utilizing its unique temperature-dependent conformation change, PNIPAM has been successfully used in drug delivery,45 separation of biomolecules,46,47 medical diagnostics,48−50 cell adhesion in tissue engineering,51 and other biological processes.40,52−54 Constructing temperature sensing devices using metallic nanostructures such as spherical gold nanoparticles,55,56 gold island films,57 gold nanorods,58,59 and lithographically designed gold nanodots60,61 and the thermoresponsive polymer PNIPAM has been problematic and displaying low sensing efficiency. The LSPR response of colloidal gold nanoparticles is significantly lower than other nanostructures.2,25 Furthermore, it is difficult to control the polymer density on the dissolved nanostructure in solution, which compromises sensing efficiency and reproducibility. Recently, lithographically fabricated gold nanodots were used for the design of a temperature

sensor.60 However, the method not only failed to control the polymer density on the nanodot surface but also showed nonspecific attachment of the polymer onto the solid supporting substrates, which would impede with the LSPR signal and result in inaccurate sensitivity. Therefore, a new fabrication strategy is required to design solid-state, ultrasensitive temperature sensors by combining highly responsive plasmonic nanostructures and suitable surface ligand chemistry to achieve the highest sensitivity and reversibility. Recently, we have demonstrated that chemically synthesized gold nanoprisms with average edge lengths of 22−52 nm that act as nanoantennas can be attached onto supporting substrates for the fabrication of ultrasensitive LSPR-based biosensors. Furthermore, we have also shown that the bulk refractive index sensitivity and figure of merit of our sensors is higher22,24 than spherical gold nanoparticles, gold nanostars, and gold nanorods because the nanoprisms possessed sharp tips with enhanced electromagnetic field (EM) and large sensing volume.24,62 These two parameters significantly influence the sensitivity of plasmonic nanosensors. In the current study, we have developed an original multistep strategy to design a highly reversible temperature sensor as outlined in Figure 1. Nanoprisms are first attached to silanized glass. Then a mixed monolayer of thiophenol (TP) and mercaptobenzoic acid (MBA) was prepared onto the nanoprism’s surface. This step was found to be critical to obtain the highest temperature sensitivity. Finally, poly(allylamine hydrochloride)-co-poly(Nisopropylacrylamide) (PAH-co-PNIPAM) was chemically attached onto the surface of the gold nanoprisms via amide coupling between the acid group present in MBA and an amine group in the polymer backbone. Most importantly, we have observed that the edge length of nanoprisms plays a crucial role in the sensitivity of the sensor and have found that larger edge length nanoprisms displayed higher sensitivity. It was observed that ∼40 nm edge length nanoprisms displayed an ∼18.2 nm LSPR peak red-shift as the temperature changed from 18 to 56 °C. To the best of our knowledge, this is the highest temperature sensitivity reported in the literature for solidstate nanosensors.

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EXPERIMENTAL SECTION Materials. Chloro(triethylphosphine)gold(I) (Et3PAuCl), poly(methylhydrosiloxane) (PMHS, Mn = 1700−3300), trioctylamine (TOA, 98%), (3-mercaptopropyl)triethoxysilane 26229

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Figure 2. (A) Scanning electron microscopy (SEM) image of gold nanoprisms on silanized glass surface. The scale bar is 100 nm. (B) Atomic force microscopy (AFM) image of the gold nanoprisms on silanized glass surface.

mixture was then placed in an oil bath at 150 °C. Separately, under nitrogen, 5 mL of chlorobenzene was added dropwise to 0.8 mol % of AIBN, and the oxygen was removed from the solution by freeze−thaw cycles as described above. The AIBN− chlorobenzene solution was then added to the PAH and Nisopropylacrylamide solution and refluxed for 3 h. The solution was cooled to 4 °C, and a solid formed. The solid was then dissolved in ethyl acetate and allowed to sit overnight. The precipitate was collected and purified by dissolving in methanol and reprecipitating in ethyl acetate. The reprecipitation process was repeated until the monomer peak of the allyl group disappeared on the 1H NMR spectra. The molecular weight of PAH-co-PNIPAM was determined by 1H NMR and found to be ∼18 000. Synthesis of Gold Nanoprisms. The gold nanoprisms, which displayed LSPR dipole peaks (λLSPR) at 700 and 780 nm, corresponding to edge lengths of ∼28 and ∼40 nm, were synthesized according to our previously published procedure.22,24 In this procedure, 20 mL of acetonitrile was used to dissolve 0.02 mmol of Et3PAuCl and was stirred at room temperature for 30 min. Next, 0.06 mL of TOA (0.137 mmol) was added to solution, followed by an additional 30 min of stirring. Then, 0.3 mL of PMHS was injected into the reaction and was stirred at room temperature for 30 min. The reaction was then heated at 40 °C for 220 min. The solution started out clear, slowly changed colors, and became pink, purple, and then dark blue. The dark blue reaction solution displayed stable λLSPR at 700 nm (Supporting Information Figure 1). The solution was then removed from heat, centrifuged, and used to fabricate temperature sensors. The ∼40 nm edge length prisms (λmax = 780 nm, Supporting Information Figure 1) were prepared under similar conditions and mole ratio of gold salt and PMHS, but used 0.032 mL of DIEA in place of TOA. Functionalization of Glass Coverslips with MPTES and Construction of Temperature Sensors. The glass coverslips (supporting substrates) were functionalized according to literature procedures.22 Coverslips were immersed in a 20% (v/v) aqueous RBS 35 detergent solution at 90 °C for 30 min, followed by 5 min of sonication. After thoroughly rinsing the coverslips with water, they were placed in a solution of hydrochloric acid and methanol (1:1 v/v) for 30 min. The coverslips were then rinsed three times with nanopure water and dried in a vacuum oven at 60 °C overnight. Then, the coverslips were immersed in a solution of 10% MPTES in ethanol for 30 min, sonicated for 5 min, and rinsed with anhydrous ethanol. The coverslips were rinsed with ethanol and

(MPTES, >80%), mercaptobenzoic acid (MBA, 99%), thiophenol (TP, 99%), 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC), N-hydroxysuccinimide (NHS), allylamine (>99%), N-isopropylacrylamide (99%), 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%), chlorobenzene, anhydrous acetonitrile (CH3CN), diethyl phosphate (95%), tert-butanol, ethyl acetate, hexanes, methanol, and ethanol were purchased from Sigma-Aldrich and were used as received. Hydrochloric acid was obtained from Acros Organics, and sodium hydroxide (NaOH) was purchased from Fisher Chemicals and used without additional purification. RBS 35 detergent was obtained from Thermo Fisher Scientific and used as received. All water was purified using a Thermo Scientific Barnstead Nanopure system. Spectroscopy and Microscopy Measurements. Absorption and extinction spectra in the range of 300−1100 nm were collected with a Varian Cary 50 Scan UV−vis spectrophotometer using a 1 cm quartz cuvette. All the absorbance spectra were collected using 0.3 mL of reaction solution diluted in 2.0 mL of acetonitrile. The temperature-dependent UV−vis extinction spectra were collected in the range of 350−900 nm using a Cary Varian 100 Bio UV−vis spectrophotometer. Scanning electron microscopy (SEM) images were acquired using a Hitachi S-4700 FESEM at 20 kV. Atomic force microscopy (AFM) images were obtained using a Bioscope AFM instrument. The AFM was operated in tapping mode using beam shaped super sharp silicon cantilevers having an average force constant of 42 N/m. The operation frequency of the cantilevers for all measurements was 330 kHz. Synthesis of PAH-co-PNIPAM Polymer. The polymer poly(allylamine hydrochloride)-co-poly(N-isopropylacrylamide) (PAH-co-PNIPAM) was synthesized via free radical polymerization of allylamine and isopropylacrylamide with a monomer feed ratio of 50:50 mol % and purified according to a previously published method.63 The poly(allylamine hydrochloride) (PAH) was synthesized by mixing 10 mL of allylamine with 6 mL of hydrochloric acid and allowed to react overnight under vacuum at 50 °C. The PAH was purified by dissolution in acetonitrile, recrystallized with ethyl acetate, and dried under vacuum. 1H NMR confirmed the formation of product. Under nitrogen atmosphere, 0.025 mol of PAH was combined under nitrogen with 0.025 mol of N-isopropylacrylamide, 0.4 mol of diethyl phosphite, and 30 mL of tert-butanol. Oxygen was removed by freezing the solution, applying high vacuum, and then returning to nitrogen. This freeze−thaw cycle was then performed an additional two times. The reaction 26230

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Figure 3. (A) Ensemble extinction spectra of nanoprisms before functionalization (black, λLSPR: 716 nm), after functionalization with mixed thiols (red, λLSPR: 740 nm), and after attachment of PAH-co-PNIPAM via amide coupling (blue, λLSPR: 756 nm). All spectra were recorded in water at 18 °C. (B) AFM image of the gold nanoprisms attached to the supporting substrate after polymer functionalization. (C) Line scans across the prism show apparent height before (black) and after (red) functionalization with thiols and then PAH-co-PNIPAM.

sonicated, which was repeated at least five times. After rinsing, the coverslips were set in a vacuum oven at 120 °C for 3 h. The MPTES-functionalized coverslips were then incubated for 30 min in a freshly prepared gold nanoprisms solution. After incubation, the substrate-bound gold nanoprisms were rinsed with ethanol, dried under nitrogen, and stored under nitrogen at 4 °C. Tape cleaning was performed on the sensing platform to remove nonprismatic nanostructures by placing the adhesive (Scotch) tape onto the gold nanoprisms attached supporting substrate, gently pressed down with a finger, and slowly removed at a 90° angle. The nanoprisms containing supporting substrates were then incubated into 1 mM ethanolic solution of 1:9 MBA:TP for overnight. Next, the thiol-functionalized gold nanoprisms were rinsed with ethanol and incubated in 0.2 M aqueous solution of EDC/NHS for 2 h, followed by overnight incubation in aqueous PAH-co-PNIPAM (1 mg/mL, Mw = 1.8 × 103) solution. The prepared temperature sensors were rinsed with copious amounts of nanopure water.

the highest temperature sensitivity, the nanoprisms containing supporting substrates were incubated in a 1 mM ethanolic solution of 1:9 mol ratio of MBA:TP overnight. The MBA concentration-dependent sensor response is also investigated and described in a later part of this article. Next, the copolymer PAH-co-PNIPAM (1.0 mg/mL) was attached to the surface of the nanoprisms through amide coupling in the presence of 0.2 M EDC/NHS by reacting for 2 h at room temperature in water. The PAH provided the reactive amine group, which coupled with acid group of MBA to form amide bond. The chemical attachment of copolymer provides additional stability of the sensor while free PNIPAM chain should have sufficient space to undergo swelling and shrinking transitions with temperature change. Figure 3A shows the UV−vis extinction spectra of the substrate bound gold nanoprisms after each functionalization step. The supporting substrate-bound nanoprisms displayed λLSPR at 716 nm in water. An ∼24 nm λLSPR red-shift was observed after mixed thiols functionalization and an additional ∼16 nm red-shift detected after attaching the polymer on nanoprism’s surface via amide coupling. The red-shift of the dipole peak after each functionalization step is in agreement with the plasmonic properties of nanoantennas that increase of the local dielectric environment around the nanostructure results in shift of the peak in the direction of longer wavelength.9,27 It can be noted that after thiol functionalization the LSPR dipole peak bandwidth increased due to the dampening; however, no further peak broadening was observed during the polymer attachment.

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RESULTS AND DISCUSSION Fabrication and Characterization of LSPR-Based Nanosensors. Our multistep strategy to fabricate a temperature-dependent nanosensor (sensing platform) is shown in Figure 1. Before thiol and polymer functionalization, the glass substrate-bound nanoprisms were analyzed by SEM and AFM. Figures 2A and 2B illustrate the SEM and AFM image of gold nanoprisms, respectively. Nearly 95% of nanostructures belonged to nanoprisms with an average edge length and height were ∼28 and ∼7.8 nm, respectively. In order to achieve 26231

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Figure 4. (A) UV−vis extinction spectra of PAH-co-PNIPAM-functionalized gold nanoprisms with an ∼28 nm edge length attached to the supporting substrate in nanopure water at 18 °C (blue, λLSPR: 756 nm) and 56 °C (red, λLSPR: 766.9 nm). (B) Experimental determination of the refractive index of the nanosensor in different media. The solid line is the calibration curve, which displays a sensitivity of 436 nm/RIU (RIU = refractive index unit) (R2 = 0.998). The refractive index of temperature sensor was determined to be 1.33 (blue dot) and 1.35 (red dot) in water at 18 and 56 °C, respectively. (C) The λLSPR shifts at two-degree intervals upon increase of solution temperature from 18 to 56 °C (blue) and reverse back from 56 to 18 °C (red) for the same thermal transducers. The inset shows the change in λLSPR shifts as a function of solution temperature for repeated cycles between 18 and 56 °C.

conformational changes of PNIPAM at temperature >30 °C increases the density of polymer around the nanoprism’s surface and increases the local refractive index.58,60,64,65 Therefore, an ∼11 nm red-shift is due to the collapse of the polymer chain, which alters the LSPR properties by increasing the refractive index around nanoprisms. In order to establish the correlation between the λLSPR shift and refractive index, one control experiment was carried out. We determined the change in refractive index around nanoantenna due to the conformational change of the thermoresponsive polymer (PNIPAM) at 18 and 56 °C. We calibrated the sensitivity of the nanosensors with respect to the change in the refractive index of the surrounding by determining the extinction spectra of the temperature sensors by exposing the sensing platforms to different solvents with varied refractive indices such as water (n = 1.33), acetonitrile (n = 1.34), ethanol (n = 1.36), and carbon tetrachloride (n = 1.46). Figure 4B illustrates the λLSPR position (blue dots) as a function of the refractive index, which provides the sensitivity of the LSPR-based nanosensors as 436 nm/RIU (RIU = refractive index unit). Based on this calibration curve, the 10.9 nm redshift of λLSPR upon changing the solution temperature from 18 to 56 °C resulted in increase of refractive index of 0.02. Previously, it was reported that detecting a similar λLSPR shift using substrate-bound gold nanodots required a refractive index

The LSPR-based nanosensors were further analyzed by AFM to measure the thickness of the polymer layer on the nanoprisms. Figure 3B represents the AFM image of the gold nanoprisms after functionalization with thiol and polymer. Clearly, the sharp features of nanoprisms become blurred after polymer attachment. Additionally, the polymer has covered the entire prisms’s surface. Most importantly, no nonspecific attachment of polymer on the glass substrate was observed. From the AFM analysis, the average height of the dielectric shell including thiol and polymer was determined to be ∼11.8 nm, which was an ∼4 nm increased in height after two functionalization steps (see Figure 3C). Therefore, the polymer thickness measured by AFM was ∼3.2 nm (the thickness of MBA is around ∼0.8 nm calculated from ChemDraw 3D). Supporting Information Figure 2 (SI-Figure 2) illustrates the histogram of high profile at different stages of nanoprisms surface functionalization as determined from AFM analyses. Investigation of the Temperature Sensitivity of LSPRBased Nanosensors. The temperature-dependent sensitivities of the LSPR-based nanosensors were investigated by monitoring the λLSPR shift as a function of solution temperature. The sensing platform was immersed in water at 18 °C (laboratory water temperature), which displayed λLSPR at 756 nm, while the solution temperature increased to 56 °C the λLSPR red-shifted to 766.9 nm, as shown in Figure 4A. The 26232

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Figure 5. (A) UV−vis extinction spectra of ∼28 nm edge length gold nanoprisms bound to the supporting substrate without (blue, λLSPR: 716 nm) and with PAH-co-PNIPAM attachment in nanopure water at 18 °C (red, λLSPR: 719 nm) and PAH-co-PNIPAM-attached nanoprisms at 56 °C (black, λLSPR: 723 nm). (B) Shift of dipole peak positions (ΔλLSPR) of gold naoprisms bound to silanized glass substrate after functionalization with different mole percentage of MBA (red bars), after attachment of PAH-co-PNIPAM via amide coupling (blue bars), and after cycling the temperature between 18 and 56 °C (green bars). All spectra were recorded in water.

change of 0.06.60 Therefore, the experimental data unequivocally prove that gold nanoprisms are more sensitive to changes in the surrounding environment and suitable for ultrasensitive nanosensors fabrication. To measure the temperature sensitivity and reversibility of our plasmonic nanosensors, the sensing platform was placed in nanopure water inside of a UV cuvette at 18 °C, and the solution temperature was slowly increased. The position of λLSPR was measured at every two-degree interval until the solution temperature reached to 56 °C. Once the solution temperature reached 56 °C, the solution temperature was gradually decreased and the position of λLSPR was measured until the temperature reached 18 °C. Figure 4C displays the λLSPR shift at two-degree intervals from 18 to 56 °C (blue squares) and back from 56 to 18 °C (red squares) for our LSPR-based nanosensors. As the temperature increased from 18 to 30 °C, the λLSPR red-shifted about ∼2 nm. Once the solution reached to LCST of 32 °C, the λLSPR shift was rapid, and an ∼8 nm red-shift was observed at 38 °C. From 38 to 56 °C, the λLSPR red-shifted to another ∼1 nm. The sharp λLSPR change in the temperature range of 30 and 38 °C is the LCST phase transition region. This transition region is lower than the spherical gold nanoparticle-based temperature sensor64 and suggests that our nanosensor has better sensing efficiency. As the solution temperature decreased, the λLSPR blue-shifted back to approximately the original wavelength at 18 °C. As the solution temperature decreases, the polymer chain extends, consequently lowering the local refractive index around nanoprisms, resulting in a blue-shift of the λLSPR. We also found that our LSPR-based nanosensors were highly reproducible and reversible. They can undergo at least 10 heating and cooling cycles without any significant changes in the LSPR properties. The inset of Figure 4C shows the reversibility of the nanosensor for several heating and cooling cycles. This result is significant and suggested that the chemical attachment of PNIPAM on the nanoantenna surface increases the durability of the newly designed temperature sensor. Surface Ligand Chemistry Dependent Temperature Sensitivity of Nanosensors. We have also investigated the effects of surface ligand chemistry on the temperature sensitivity of our LSPR-based nanosensors. It is known that primary amines adsorb onto the gold nanoparticle surface

through electrostatic interaction.66,67 Therefore, we would expect that the amine group in the PAH tail from PAH-coPNIPAM would interact with nanoprisms and adsorb onto its surface. The glass substrate-bound gold nanoprisms were incubated in a 1.0 mg/mL aqueous solution of PAH-coPNIPAM for overnight and followed by washing with copious amounts of water. An ∼3 nm λLSPR red-shift was observed in the UV−vis spectrum after the polymer adsorption onto the nanoprism surface. This value is nearly 5 times lower in comparison to the λLSPR shift (∼16 nm) observed when PAHco-PNIPAM was covalently attached to the nanoprism’s surface. This result suggests that the amount of polymer on the nanoprism’s surface was significantly low, and covalent attachment of polymer could be the appropriate way to increase the polymer concentration onto the nanoprism surface. Figure 5A illustrates the extinction spectra of polymer adsorbed gold nanoprisms onto supporting substrates. The polymeradsorbed nanoprisms were then tested for temperature sensitivity, and as we expected, an ∼4 nm λLSPR red-shift was observed upon changing the solution temperature from 18 to 56 °C (see Figure 5A). We believe this low sensitivity of the nanosensor is due to the inadequate amount of polymer bound to nanoprisms surface. Previously, we have discussed that 10 mol % of MBAfunctionalized LSPR-based nanosensors displayed an ∼11 nm λLSPR red-shift upon increase of solution temperature from 18 to 56 °C. We further investigated the effects of surface ligand chemistry on the sensing performance of our newly designed temperature sensor. The gold nanoprisms surface was functionalized with a mixture of MBA and TP in different mole ratio of MBA, 100, 75, 50, and 25 mol %. Figure 5B illustrates the histogram of λLSPR red-shift (red bars) in the presence of different mole ratio of MBA. Interestingly, ∼39 and ∼24 nm λLSPR shifts were observed when nanoprisms were functionalized with 100 and 10% MBA, respectively. The MBA concentration dependent large LSPR peak shift of nanoprisms is unique, which could be due to the electron-withdrawing ability of the acid group from MBA. This unusual phenomenon of LSPR peak shift is currently under investigation. Nevertheless, we could expect that increase of the MBA concentration on the nanoprism surface would subsequently increase the amount of PAH-co-PNIPAM due to a higher 26233

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Figure 6. (A) AFM image of the gold nanoprisms attached onto silanized glass substrate without any surface modification. (B) AFM image of the gold nanoprisms attached onto supporting substrate after thiol functionalization and polymer attachment.

Figure 7. (A) λLSPR at two-degree intervals from 18 to 56 °C (blue) and back from 56 to 18 °C (red) for gold nanoprisms with ∼40 nm edge length functionalized with PAH-co-PNIPAM. Inset: variation in λLSPR of PAH-co-PNIPAM-functionalized gold nanoprisms as a function of temperature for repeated temperature cycles from 18 to 56 °C. (B) Ensemble UV−vis spectra of thermoresponsive polymer-functionalized gold nanoprisms with ∼42 nm edge length attached to the supporting substrate in water at 18 °C (blue, λLSPR: 831 nm) and 56 °C (red, λLSPR: 849 nm).

number of acid groups available for amide coupling. However, Figure 5B shows that λLSPR red-shifts (blue bars) are nearly similar when PAH-co-PNIPAM was covalently attached to a nanoantenna surface containing different mole percentages of MBA. We believe that even at a low MBA concentration (10 mol %), the surface of the nanoprism was fully covered with PAH-co-PNIPAM and formed a single polymer layer. The temperature sensitivities of sensing platforms containing different MBA concentrations on their surface were also investigated by placing them in water. Interestingly, the presence of a higher concentration of MBA exhibited the lowest λLSPR red-shifts of ∼5 nm, whereas a lower concentration of MBA demonstrated ∼11 nm shifts (green) when the water temperature was raised to 56 from 18 °C. Since the λLSPR redshifts were nearly identical for various concentrations of MBA present on the surface of nanoprisms that were further functionalized by PAH-co-PNIPAM, we would expect similar temperature sensitivity considering the polymer concentration is similar on the nanoprism surface. The low temperature sensitivity of the LSPR-based nanosensor, which was prepared in the presence of a higher concentration of MBA, restricted

the shrinking and swelling transitions of the polymer. The PAH has a −NH2 group in their tail, and PNIPAM has a −NH group in the backbone. Therefore, with large excesses of free −COOH groups present on the nanoprism surface from MBA, the acid and amine groups form hydrogen bonds in water. Most likely, multiple hydrogen bonds prevent the PNIPAM chains to undergo temperature dependent shrinking and swelling transitions, which result in small changes in the local dielectric environment of the nanoantenna and low temperature sensitivity of the nanosensor. Gold Nanoprisms Edge Length Dependent Temperature Sensitivity. Recently, we have shown that sensing volume of gold nanoprisms plays an important role in their refractive index and biosensing abilities.24 Importantly, we have observed that the sensitivity due to the changes in local refractive index is proportional to the sensing volume of the nanoprisms. Since the temperature-induced phase transition of PNIPAM alters the local refractive index, we expect that longer edge-length nanoprisms display a large change in λLSPR as a function of solution temperature. In order to investigate actual performance of the plasmonic nanosensors, we studied the 26234

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nanoprism’s edge length dependent λLSPR shift as a function of temperature with gold nanoprisms with ∼40 nm edge length. The gold nanoprisms with ∼40 nm edge lengths were synthesized according to our previously published procedures (see Experimental Section for details). The attachment of ∼40 nm edge length nanoprisms on silanized glass surface and then PAH-co-PNIPAM functionalization were identical as described for the ∼28 nm edge length, where a 1:9 mole ratio of MBA to TP was used. The different functionalization steps were characterized by UV−vis spectroscopy (see Supporting Information Figure 3) and AFM. Figures 6A and 6B represent the AFM images of the ∼40 nm edge length gold nanoprisms before and after the thiols and polymer functionalization, respectively. Supporting Information Figure 2 illustrates the histogram of height profile of nanoprisms before surface modification (green) and after (black) mixed thiol functionalization followed by polymer attachment on the nanoprisms surface. The nonfunctionalized nanoprisms have an average height of 9.8 nm, which increased to ∼13.7 nm after all the functionalization steps. The ∼3.9 nm height differences belong to the height of MBA and TP layer, and the thickness of the PAH-co-PNIPAM polymer, which resulted a polymer thickness of ∼3.1 nm. This polymer thickness is consistent with our previous analysis (3.2 nm thick) in which sensing platforms were prepared with ∼28 nm edge-length nanoprisms. The AFM image of PAH-coPNIPAM-functionalized LSPR-based nanosensors showed that the polymer covered the entire nanoprisms surface and their sharp edges become rounded; however, a triangular shape of the polymer-covered nanoprisms was clearly visible. The sensitivity of LSPR-based nanosensors prepared with ∼40 nm edge length nanoantennas was investigated by placing them in water and increasing the temperature at two-degree intervals from 18 to 56 °C. As shown in Figure 7A, it was found that only an ∼3 nm red-shift was noticed until the temperature reached 32 °C, but an ∼11 nm red-shift occurred from 32 to 40 °C; another 3 to 4 nm shift was detected from 40 to 56 °C. A total λLSPR red-shift of ∼18.2 nm was observed as the solution temperature increased from 18 to 56 °C. The sensors showed a slightly different trend in LSPR response at temperatures greater than and less than 34 °C as the solution temperature was reversed back from 56 to 18 °C. Figure 7B illustrates the UV−vis spectra of a thermoresponsive polymer-functionalized nanosensor at 18 °C (blue, λLSPR = 831 nm) and at 56 °C (red, λLSPR = 849 nm). Figure 7A inset displays the reversibility of the gold nanoprisms for several heating and cooling cycles, which indicates that the temperature responsive nanosensors are very stable and reversible. For the first time we have shown that the temperature dependent sensing efficiency of a LSPR-based nanosensor strongly depends on the sensing volume of the nanoantennas. We have shown that the unique swelling−shrinking phase transition of PNIPAM can effectively alter the local dielectric environment of nanoprisms resulting in alteration of their LSPR properties. Even though PNIPAM is the most widely used polymer for temperature-induced LSPR-based sensor development, its narrow LCST (32 °C) makes the sensors responsive over only a limited temperature range. To overcome this narrowness problem, other functional homo- or copolymers68 can be used in nanosensor design to expand the sensing range from 15 to 80 °C and thus to increase the versatility of the sensors. Furthermore, our study using two different edge length nanoprisms is a proof-of-concept, which shows that

sensing volume of metallic nanostructures is an important parameter in enhancing device sensitivity. Therefore, in addition to nanoprisms, gold nanorods would be a good candidate for fundamental studies and future device fabrication, since the latter nanostructures can be prepared in wide variety of lengths and widths and provide a large range of sensing volume.69−71 The LSPR-based temperature sensing demonstrated in this article has many advantages over existing systems such as: (1) the LSPR peak shift observed here is achieved from a thin layer of polymer (∼3.2 nm thickness) rather than thick polymeric film (>150 nm),60 (2) there is highly specific attachment of polymer on the surface of the nanoantennas as opposed to other methods in which polymers were uncontrollably bound onto both nanostructure and supporting substrates,60 and (3) unlike other temperature sensors, which only operate in solution,58,64,72,73 the PAH-co-PNIPAM-functionalized substrate-bound gold nanoprisms work in solid state. The surface ligand chemistry used for the fabrication of nanosensors can be tuned to prepare desired chemical functionalities onto the surface of nanoantennas. This will allow us to attach various stimuli-responsive molecules on them, which will alter the optical signal of nanoantennas upon variation of the wavelength of light, solution pH, and chemical reagents.

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CONCLUSIONS

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ASSOCIATED CONTENT

Temperature dependent solid-state plasmonic nanosensors have been designed using the thermoresponsive polymer PAH-co-PNIPAM-functionalized gold nanoprisms. When the solution temperature was increased from 18 to 56 °C, the 28 and 40 nm edge length gold nanoantennas displayed average λLSPR peak shifts of 10.9 and 18.2 nm, respectively. The observed λLSPR shift belongs to the extended and the collapsed conformational state of PNIPAM below and above the LCST, respectively. The temperature dependent λLSPR shift we observed for LSPR-based solid-state nanosensors prepared with gold nanoprisms is higher than previously reported nanoassemblies with PNIPAM and spherical gold nanoparticles (∼6 nm),74 PNIPAM brushes immobilized with gold nanoparticles (∼12 nm),55 NIPAM-co-GMA-functionalized gold nanoparticle (∼11 nm),75 or lithographically fabricated nanodots (∼10 nm).60 Our highest sensitivity of 18 nm λLSPR shift is lower than PNIPAM microgel-coated nanorod (∼28 nm).58,59 However, stability of dispersed nanostructures in solution at higher temperature was problematic and resulted in precipitation, which was found to be difficult to redissolve in solution.58 For the first time the dimension dependent temperature sensitivity of nanostructures has been studied. The nanoprisms with a longer edge length displayed a higher sensitivity compared to smaller edge length gold nanoprisms, indicating that the sensitivity is highly size dependent where sensing volume plays a critical role. In addition, the reversibility of the temperature dependent nanosensor suggests that the sensors are very stable and could potentially be used as reversible temperature switches in real-time nanosensing applications and also in bioengineering.

S Supporting Information *

UV−vis absorption spectra of different edge-length nanoprisms in acetonitrile, additional extinction spectra, and a histogram illustrating the height measurements at different stages of 26235

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nanoprisms surface functionalization. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Start-up funds provided by IUPUI. We also thank Amar Kumbhar (CHANL, UNC Chapel Hill) for helping with SEM analysis. K.A.S. acknowledges a Robert Welch summer fellowship from the Department of Chemistry and Chemical Biology at IUPUI.

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