Tricomponent Multiblock Nanorods for Fourier Transform Surface

Letter Cite This: Anal. Chem. XXXX, XXX, XXX−XXX

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Tricomponent Multiblock Nanorods for Fourier Transform Surface Plasmon Resonance and Its Chemical Sensing Insub Jung, Seongkeun Ih, Sangbaek Jung, Sungjae Yoo, and Sungho Park* Department of Chemistry, Sungkyunkwan University, Suwon, Gyeonggi 16419 South Korea

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S Supporting Information *

ABSTRACT: In this study, we report a new mode of chemical sensing using Fourier transform surface plasmon resonance with tricomponent nanorods (Au, Ni, and Pt). By applying an external magnetic field, magnetically responsive multiblock nanorods fluctuate periodically, producing sigmoidal optical responses that are represented as a dominant frequency peak after Fourier transform conversion. Adding H2O2 to the solution under an external magnetic field perturbed the periodic nanorod rotation due to a catalytic reaction between the Pt segment and H2O2, which produces catalytic random fluctuation states. The target chemicals were detected by measuring the frequency domain recovery time between two competing states, the magnetic dominant state and the catalytic random state. These two states can be controlled and maximized by nanorod block design, demonstrating the effectiveness of our chemical sensing design using Fourier transform surface plasmon resonance.

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effective sensing platform for target analytes. The FTSPR led to an overall platform sensory increase due to background signal reduction using a simple transformation modality. In this study, we demonstrate a new method in which the FTSPR process can be extended to chemical sensing. A schematic illustration of the concept and the experimental setup are shown in Figure 1A. First, using an electrochemical deposition method within porous anodized aluminum oxide templates (AAO), we synthesized tricomponent multiblock nanorods containing a Pt segment at each end and two Au plasmonic blocks and magnetically active Ni segments in the middle (see the Supporting Information for further experimental details). Each Pt segment plays a key role, catalyzing H2O2 (i.e., electrophoresis) to enable propulsion of multiblock nanorods.13 Plasmonically active Au segments are used to synthetically stabilize the entire nanostructure and to produce strong sensing signals by scattering the incoming light. The Ni blocks are magnetically responsive to the external magnetic stirrer and are dominant for effective periodic rotation of the nanorods. By externally rotating the magnetic field at a certain frequency, the multicomponent nanorods rotate at the same speed as the magnetic field, producing sigmoidal functions in the time domain. Using a fast Fourier transform, the periodic signals are converted into a frequency peak that indicates the dominant rotational frequency of the multiblock nanorods. H2O2 is detected by measuring the time difference between the two competitive states caused by the magnetic field and H2O2 addition. When we apply a rotational magnetic field, it creates a magnetic dominant state with a main frequency peak,

anomaterials have been researched intensively over the past decades due to their unique electrical, optical, and catalytic properties. An exceptionally high nanomaterial surface to volume ratio enables a high sensitivity toward local environmental changes and leads to its widespread use in chemical sensors and biosensors.1,2 Among various electrical,3 electrochemical,4 and DNA-based5 sensors, optical sensors provide the opportunity for a highly sensitive and accurate optical readout as a noninvasive detective technique due to a remote light usage.6 Specifically, plasmonic nanomaterials such as noble metals (e.g., Au, Ag, and Cu) have been utilized as nanotransducers in optical sensors. The localized surface plasmon resonance (LSPR), a collective oscillation of free electrons in metallic nanomaterials under light irradiation, promotes enhanced sensitivity compared to conventional spectrometric methods (e.g., absorption, transmission, reflection, and fluorescence). Since LSPR is very sensitive to nanomaterial shape, size, and surrounding media, many researchers have tried to maximize plasmonic enhancement by synthesizing complex nanostructures (either single or assembled) to enhance local electric fields or by constructing new sensing configurations.7−9 In a previous study, we reported Fourier transform surface plasmon resonance (FTSPR),10−12 where an external magnetic field induces repeated magnetic modulation of plasmonicmagnetic multiblock nanorods in solutions and, consequently, generates a sigmoidal optical signal that is represented as a dominant frequency signal after Fourier transform conversion. As the plasmonic-magnetic nanorods periodically fluctuate under the external magnetic field, drag torques occur mainly on the edges of the nanorods and trigger abnormal behavior for target molecule attachment. This causes the frequency peaks to be shifted sequentially (i.e., frequency reduction) depending upon the concentration of target molecules, producing an © XXXX American Chemical Society

Received: January 25, 2019 Accepted: April 17, 2019 Published: April 17, 2019 A

DOI: 10.1021/acs.analchem.9b00466 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

Figure 1. Schematic illustration of the FTSPR process and experimental setup. Multicomponent nanorods are designed for chemical, plasmonic, and magnetic reactions.

Figure 2. Synthesis of multicomponent nanorods and magnetic modulation. (A) Field emission scanning electron microscopy (FE-SEM) image and EDS mapping images of Au/Ni/Au−Pt nanorods, confirming the existence of each component (Pt, Au, Ni). Optical responses (B) in the time domain and (C) in the frequency domain, indicating successful magnetic modulation of the multicomponent nanorods.

indicating that the nanorods are following the speed of the external magnetic field (denoted as an initial state in Figure 1B). This frequency peak is damped upon H2O2 exposure, and the magnetic rotation state is perturbed by catalytic reaction. Thus, the catalytic force is dominant and represents only noise-like

signals due to the random catalytic propulsion from H2O2 oxidation on the surface of the Pt segment (i.e., a damped state). After a certain amount of time under a constant magnetic field, the magnetic force again dominates the system, and the previously suppressed frequency returns to its original state (i.e., B

DOI: 10.1021/acs.analchem.9b00466 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

rotational fluctuation was perturbed by the random catalytic movement of the nanorods (the damped state). When two different reactions competed (i.e., magnetic rotation and catalytic propulsion), the FFT frequency rebounded (Δt) after 18 s, which we used as our core sensing platform. We plotted the external magnetic field (8.33 Hz) recovery time as a function of H2O2 concentration (Figure 3B). The FFT peak at 16.67 Hz remained after 60 s (Figure 3A), and there were noise-like signals in the low-frequency regime. This was attributed to microscale bubbles created during UV measurement, indicating lack of completion of the catalytic reaction. Therefore, it is improbable that the reactions occur sequentially (i.e., rotational magnetic behavior returns after the catalytic reaction is complete). Rather, it is a competitive process between the rotational magnetic and random catalytic behaviors of nanorods under specific controlled conditions. These conditions depend on the Au/Ni ratio (i.e., magnetic responsiveness), external magnetic field strength, and concentration of H2O2. When we synthesized nanorods that had a small Ni segment, no rebound state appeared until after 60 s of catalytic reaction because the small amount of Ni segments could not effectively overcome the random catalytic fluctuations (Figure S4A,C). Nanorods with long Ni segments were not affected by the addition of H2O2 due to the strong magnetic responsiveness of the Ni blocks (Figure S4B,D). We observed physical agitation of the nanorods by investigating how gas released from the Pt surface affects the FFT spectra as opposed to using conventional observations through optical microscopy. Additionally, magnetic dipoleinduced aggregation could be a primary concern; thus, we measured the kinetic assay under H2O2 injection with and without applying a magnetic field, which indicates aggregation or a sinking effect of the nanorods could be minimized (Figure S5). To gain insight on the relationship between block design and H2O2 concentration, we varied the lengths of the Pt segments from 250 to 350 and 500 nm at a fixed Au/Ni ratio. As the length of the Pt segment increases, the exposed surface area of Pt that can react with H2O2 increases,14 resulting in an increased length of the catalytic-dominant state. Thus, detectable H 2 O 2 concentration decreased as segment length increased (Figure 4A). Of critical importance is whether the Pt segment block design is correlated to sensitivity. In our previous study, we found that segment block design strongly affected nanoscale sensitivity to the local environment.10,15,16 To investigate, we designed two sets of nanorods, one with a single-catalytic Pt domain and another with a dual-catalytic Pt domain. The first has only one Pt segment located on one end, while the second has two Pt segments, one at each end of the nanorod (Figure 4B). Since catalytic propulsion of the dual-ended Pt segment with H2O2 occurs in two active catalytic blocks, it causes more perturbation in the magnetic rotational system compared to the single-Pt segment nanorods, leading to increases in recovery time and sensitivity. In comparison to the single-domain nanorods, the dual-domain nanorod (see the schematic description in Figure 4B, samples a to b and c to d) recovery time increased with the same amount of H2O2, and the detectable H2O 2 concentration decreased (Figure 4C). Specifically, the detection limit for the nanorods in sample b was much lower, 0.27 wt %, and 10 times higher than that of the nanorods in sample a (3 wt %). This is comparable to the results obtained using the electrochemical method (Figure S6). As described in Figure S4, the rotational behavior of the nanorods in sample c (i.e., long Ni segment) was not affected by the

a rebounded magnetic modulation). By measuring the time difference (Δt, recovery time) between the damped state and the rebounded state, H2O2 can be determined as a function of concentration. Representative tricomponent Au/Ni/Au−Pt nanorods are shown in Figure 2A. The diameters of the nanorods were 250 nm (the pore sizes of the AAO templates), and the total lengths of the nanorods were approximately 3.5 μm, with 1 μm of Au and Ni and 350 nm of Pt. Each component was confirmed using an energy-dispersive spectroscopy (EDS) mapping image, as shown in Figure 2A. Using an external rotating magnetic field, the multicomponent nanorods rotate at the speed of the magnetic field, producing a sigmoidal function in a time domain. Its optical response was plotted as a function of magnetic field speed (from 0.25 to 8.33 Hz) (Figure 2B). Using a fast Fourier transform, the dominant frequency peaks shifted as a function of the applied magnetic field, which indicates successful nanorod magnetic modulation (Figure 2C). To see the effect from the Au and Ni segments, we synthesized Au/Ni nanorods (i.e., no Pt segments) and measured their optical responses under a magnetic field and H2O2 exposure (Figure S1). Without magnetic field application, there was no sigmoidal function based on H2O2. Periodic fluctuations appeared when we applied a magnetic field (0.83 Hz), but there were no significant sinefunction changes as a function of H2O2 concentration (from 0 to 100 μL). This indicates that the pure Au/Ni nanorod itself does not promote a catalytic reaction affecting rotational dynamics. Au/Pt nanorods with different Au/Pt ratios were measured using H2O2 injection as a control group (Figure S2), representing only the random fast Fourier transform (FFT) spectra. From this experiment, all the rotational frequency peaks were generated from magnetic modulation of Ni segments. Figure 3A shows a representative FTSPR sensing method using the nanorods shown in Figure 2 and an injection

Figure 3. Representative H2O2 detection sensing method based on Fourier transform surface plasmon resonance: (A) Fourier transform spectra of tricomponent nanorods to an external rotating magnetic field (8.33 Hz). After adding H2O2, we observed the change of state, clearly represented in the frequency domain. (B) The plot of recovery time as a function of H2O2 concentration.

containing 3 wt % H2O2 (see Figure S3 for the raw optical response in the time domain). To achieve a rebound state, we set the magnetic field speed to 8.33 Hz (the maximum speed in our device). Initially, a frequency peak at 16.67 Hz appeared due to the two-order rotational symmetry (ω = 2πf) (the initial state, the bottom spectrum of panel A). As H2O2 was added, the FFT frequency peak at 16.67 Hz disappeared, indicating that periodic C

DOI: 10.1021/acs.analchem.9b00466 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

in the frequency domain, we measured the target chemicals. The magnetic rotational states and catalytic random states can be tuned by controlling the length, position, and number of catalytic Pt segments and are correlated to the detectable amount of H2O2. This methodology contributes to our understanding of diverse nanoscale fluctuations caused by multiple external sources and can be applied to micro/ nanorobotics and diverse sensor fields.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.9b00466.

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Experimental section, FTSPR measurements using Au/Ni and Au/Pt NRs, electrochemical measurement of H2O2 on Au/Pt-array films, kinetic assay under magnetic field, and sensitivity comparison of multiblock nanorods (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 4. Nanoblock design comparing sensitivity to H 2 O 2 concentration. (A) H2O2 sensing showing that the detectable H2O2 concentration decreases as Pt length increases. (B) Illustrations of nanorods composed of single- and dual-Pt domains that are responsible for damping states. (C) Corresponding H2O2 sensing indicating that, when a Pt segment is attached to each side of a nanorod, magnetic movement is more perturbed compared to Pt segments attached on only one side.

ORCID

Sungho Park: 0000-0001-5435-3125 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the Korea National Research Foundation (Research Fellow program: NRF-2018R1A6A3A11049008). This research was also supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (Grant No. NRF-2017R1A2B3002796) and the National Research Foundation of Korea (NRF) grant funded by the Korea governmnt (MSIT) (No. NRF-2017R1A5A105365).

surrounding catalytic reaction throughout the entire measurable H2O2 concentration range due to the strong magnetic response (black dots in Figure 4C). We measured the recovery time between the damped states and the rebounded states (see the red line in Figure 4C) for the nanorods with the dual Pt domains (nanorod sample d). The two competing states (i.e., magnetic rotational behavior and catalytic random behavior) can be tuned by controlling the appropriate segment block design, which will eventually lead to a smaller detectable amount of H2O2. We also investigated the number of active catalytic blocks by increasing the number of Pt segments at a fixed total sum of Pt lengths (Figure S7). Compared to nanorods with Pt segments embedded between Au and Ni blocks, nanorods with a Pt segment at each end were optimal for perturbing the magnetic rotational equilibrium as they caused the highest random torques. In this work, we focused on whether our FTSPR could be extended to chemical sensing. Considering our previous works for detection of target biomolecules, we believe this method can be more general under appropriate block design (i.e., the number of blocks, block components, total lengths, relative segment lengths) or consider if we can induce two competitive fluctuation states since there are many external stimuli such as light, magnetic field, ultrasound, etc. In conclusion, we proposed a new way to detect target chemicals using tricomponent Au/Ni/Au−Pt nanorods combined with an FTSPR application. By applying an external magnetic field, magnetic nanorods fluctuated constantly, allowing them to be analyzed using a Fourier transform. When H2O2 was added to the solution, the magnetic rotation of the nanorods was perturbed, leading to random catalytic fluctuation. By measuring the recovery time between two fluctuation states

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DOI: 10.1021/acs.analchem.9b00466 Anal. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.analchem.9b00466 Anal. Chem. XXXX, XXX, XXX−XXX