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Bioinspired Brochosomes as Broadband and Omnidirectional Surface-Enhanced Raman Scattering Substrates Qianqian Ding, Yanlei Kang, Wanlin Li, # Guofang Sun, § Hong Liu, Ming Li, Δ Ziran Ye, § Min Zhou,* ,# Jianguang Zhou,* ,and Shikuan Yang* ,Institute for Composites Science Innovation, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China State Key Laboratory of Industrial Control Technology, College of Control Science and Engineering, Zhejiang University, Hangzhou 310027, China # The Second Aliated Hospital, School of Medicine, Zhejiang University, Hangzhou 310009, China § Department of Applied Physics, College of Science, Zhejiang University of Technology, Hangzhou 310014, China Δ State Key Laboratory for Power Metallurgy, School of Materials Science and Engineering, Central South University, Hunan 410083, China * S Supporting Information ABSTRACT: Surface-enhanced Raman scattering (SERS) substrates capable of working under laser excitation in a broad wavelength range are highly desirable in diverse application elds. Here, we demonstrate that the bioinspired Ag brochosomes, hollow microscale particles with submicroscale pits, have broadband and omnidirectional SERS performance. The SERS performance of the Ag brochosomes under near-infrared laser excitation makes them promising for applications in biosensing elds, such as the sensitive detection of Staphylococcus aureus bacteria and bovine hemoglobin protein. Additionally, the SERS intensity was insensitive to the incident angle of the laser beam, resulting from the spherical structure of the Ag brochosomes. The omnidirectional SERS performance makes the Ag brochosomes have application potential for in-the-eld analysis using a hand-held Raman spectrometer for which it is dicult to accurately control the laser beam normal to the SERS substrates. Overall, the broadband and omnidirectional brochosome SERS substrates will nd applications in diverse elds, particularly in biomedicine and in-the-eld analysis. S urface-enhanced Raman scattering (SERS) has promising applications in sensing elds, owing to its single-molecule- level detection capability, strong specicity (i.e., providing ngerprint signals of analytes), and water-inactiveness endowed biocompatibility. 17 Electromagnetic enhancement is the main origin of SERS, originating from the localized surface plasmon resonance (LSPR) of metallic nanostruc- tures. 8 The location of the LSPR peak should be adjusted to match the laser wavelength used for SERS measurements to attain strong SERS enhancement. 911 It is challenging to precisely manipulate the LSPR peak of the metallic nanostructures experimentally, even though many theories can predict the relationship between the LSPR location and the structure/composition of the metallic structures. 1214 It is even more dicult to shift the LSPR peak of metallic nanostructures to match the long wavelengths (e.g., 785 or 1064 nm) commonly used in in vivo SERS biosensing to achieve a deep penetration depth within tissues. 1521 In theory, a specic metallic nanostructure has to be designed for a given laser wavelength to achieve the best SERS enhancement. Dynamically modulating the LSPR peak position of metallic nanostructures has been studied extensively, but it requires complex fabrication procedures and has severe limitations in the tunable range of the LSPR peak positions. 2227 Inspired by the leafhopper-secreted brochosomes, hollow microscale granules with nanoscale indentations, we have engineered silver (Ag) brochosomes with wide-angle and ultra- antireective properties in the wavelength range of 2502000 nm. 28 This encouraged us to study their SERS performance at dierent laser excitation wavelengths. We revealed that the synthetic Ag brochosomes as a SERS substrate could work in an ultrabroad wavelength range. Good SERS performance was observed employing 532, 633, 785, and 1064 nm laser excitation. Moreover, the SERS performance of the Ag brochosomes demonstrated a strong tolerance for the incident angle of the laser beam, facilitating in-the-eld applications using a hand-held Raman spectrometer with poor control over the incident direction of the laser beam. 2931 The Ag brochosome SERS substrates will have promising applications in biological elds where long-wavelength laser excitation is Received: August 14, 2019 Accepted: October 7, 2019 Published: October 7, 2019 Letter pubs.acs.org/JPCL Cite This: J. Phys. Chem. Lett. 2019, 10, 6484-6491 © XXXX American Chemical Society 6484 DOI: 10.1021/acs.jpclett.9b02380 J. Phys. Chem. Lett. 2019, 10, 64846491 Downloaded via CENTRAL SOUTH UNIV on October 11, 2019 at 11:05:27 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Bioinspired Brochosomes as Broadband and OmnidirectionalSurface-Enhanced Raman Scattering SubstratesQianqian Ding,† Yanlei Kang,‡ Wanlin Li,# Guofang Sun,§ Hong Liu,† Ming Li,Δ Ziran Ye,§

Min Zhou,*,# Jianguang Zhou,*,‡ and Shikuan Yang*,†

†Institute for Composites Science Innovation, School of Materials Science and Engineering, Zhejiang University, Hangzhou 310027,China‡State Key Laboratory of Industrial Control Technology, College of Control Science and Engineering, Zhejiang University,Hangzhou 310027, China#The Second Affiliated Hospital, School of Medicine, Zhejiang University, Hangzhou 310009, China§Department of Applied Physics, College of Science, Zhejiang University of Technology, Hangzhou 310014, ChinaΔState Key Laboratory for Power Metallurgy, School of Materials Science and Engineering, Central South University, Hunan410083, China

*S Supporting Information

ABSTRACT: Surface-enhanced Raman scattering (SERS) substrates capable of workingunder laser excitation in a broad wavelength range are highly desirable in diverseapplication fields. Here, we demonstrate that the bioinspired Ag brochosomes, hollowmicroscale particles with submicroscale pits, have broadband and omnidirectional SERSperformance. The SERS performance of the Ag brochosomes under near-infrared laserexcitation makes them promising for applications in biosensing fields, such as the sensitivedetection of Staphylococcus aureus bacteria and bovine hemoglobin protein. Additionally,the SERS intensity was insensitive to the incident angle of the laser beam, resulting fromthe spherical structure of the Ag brochosomes. The omnidirectional SERS performancemakes the Ag brochosomes have application potential for in-the-field analysis using ahand-held Raman spectrometer for which it is difficult to accurately control the laserbeam normal to the SERS substrates. Overall, the broadband and omnidirectionalbrochosome SERS substrates will find applications in diverse fields, particularly inbiomedicine and in-the-field analysis.

Surface-enhanced Raman scattering (SERS) has promisingapplications in sensing fields, owing to its single-molecule-

level detection capability, strong specificity (i.e., providingfingerprint signals of analytes), and water-inactivenessendowed biocompatibility.1−7 Electromagnetic enhancementis the main origin of SERS, originating from the localizedsurface plasmon resonance (LSPR) of metallic nanostruc-tures.8 The location of the LSPR peak should be adjusted tomatch the laser wavelength used for SERS measurements toattain strong SERS enhancement.9−11 It is challenging toprecisely manipulate the LSPR peak of the metallicnanostructures experimentally, even though many theoriescan predict the relationship between the LSPR location andthe structure/composition of the metallic structures.12−14 It iseven more difficult to shift the LSPR peak of metallicnanostructures to match the long wavelengths (e.g., 785 or1064 nm) commonly used in in vivo SERS biosensing toachieve a deep penetration depth within tissues.15−21 In theory,a specific metallic nanostructure has to be designed for a givenlaser wavelength to achieve the best SERS enhancement.Dynamically modulating the LSPR peak position of metallicnanostructures has been studied extensively, but it requires

complex fabrication procedures and has severe limitations inthe tunable range of the LSPR peak positions.22−27

Inspired by the leafhopper-secreted brochosomes, hollowmicroscale granules with nanoscale indentations, we haveengineered silver (Ag) brochosomes with wide-angle and ultra-antireflective properties in the wavelength range of 250−2000nm.28 This encouraged us to study their SERS performance atdifferent laser excitation wavelengths. We revealed that thesynthetic Ag brochosomes as a SERS substrate could work inan ultrabroad wavelength range. Good SERS performance wasobserved employing 532, 633, 785, and 1064 nm laserexcitation. Moreover, the SERS performance of the Agbrochosomes demonstrated a strong tolerance for the incidentangle of the laser beam, facilitating in-the-field applicationsusing a hand-held Raman spectrometer with poor control overthe incident direction of the laser beam.29−31 The Agbrochosome SERS substrates will have promising applicationsin biological fields where long-wavelength laser excitation is

Received: August 14, 2019Accepted: October 7, 2019Published: October 7, 2019

Letter

pubs.acs.org/JPCLCite This: J. Phys. Chem. Lett. 2019, 10, 6484−6491

© XXXX American Chemical Society 6484 DOI: 10.1021/acs.jpclett.9b02380J. Phys. Chem. Lett. 2019, 10, 6484−6491

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needed and in-the-field sensing fields where a hand-heldRaman spectrometer is used.The Ag brochosomes were prepared by a template-based

seed-mediated electrochemical growth process as reportedbefore, as illustrated in Figure 1a. First, a monolayer colloidalcrystal (MCC) template composed of hexagonally closelypacked polystyrene (PS) spheres was prepared by a self-assembly process (see the SEM image in Figure 1b). Then, athin layer of gold (Au) film was thermally evaporated onto theMCC template behaving as the seed layer during thesubsequent electrochemical deposition process. Consequently,another MCC template composed of small PS spheres wastransferred onto the Au-covered MCC template, giving rise tothe formation of a double-layer colloidal crystal (DCC)template (Figure 1c). Ag was subsequently electrodepositedonto the MCC template, which grew exclusively at the Au-covered area and around the top of small PS spheres (Figure1a). After dissolving PS spheres in dichloromethane, a large-area Ag brochosome array was obtained (Figure 1d).To study the influence of the pit size and pit depth on the

SERS performance, we prepared Ag brochosomes with adiameter of 2 μm but with varied pit size and pit depth (Figure2). The pit depth was controlled by the electrodeposition time.The pit depth was predicted using the following equation: h =

Rt − R r( )t2 2− , where Rt is the radius of the top layer PS

spheres and 2r is the opening size of the pits. The pit size wasrelated to the size of the top layer PS spheres adopted. PSspheres with a diameter of 200, 350, and 500 nm were used tocreate the pits over the Ag brochosomes (Figure 2). The pitnumber reduced as the size of the PS spheres in the top layerMCC template increased. The opening size (calculated depth)of the pits when using 200 nm sized PS spheres afterelectrodeposition for 30, 60, and 90 s was around 130 nm (25nm), 150 nm (35 nm), and 170 nm (50 nm), respectively.When 350 nm sized PS spheres were employed, the openingsize (calculated depth) of the pits after electrodeposition for30, 60, and 90 s was 190 nm (30 nm), 230 nm (45 nm), and270 nm (65 nm), respectively. After electrodeposition for 30,

60, and 90 s employing 500 nm sized PS spheres, the openingsize (calculated depth) of the pits was 250 nm (35 nm), 327nm (61 nm), and 408 nm (105 nm), respectively. The large-area brochosome array with a uniform structure is shown inFigure S1.The SERS performance of the Ag brochosomes with

different structures was evaluated using 100 nM Rhodamine6G (R6G) aqueous solutions under 532 nm laser excitation(Figure 3). The SERS intensity varied for the Ag brochosomesprepared at different electrodeposition times. When theelectrodeposition time was 60 s using the DCC templateformed by 2 μm PS spheres at the bottom and 500 nm PSspheres at the top (referred as 500 nm/2 μm DCC template)corresponding to a pit depth of ∼61 nm, the SERS signals werethe strongest (Figure 3a). When using 350 nm/2 μm and 200nm/2 μm DCC templates, 60 s was also the best electro-

Figure 1. (a) Schematic of the fabrication process of the Ag brochosomes. Process I: Electrodeposition of Ag onto the DCC template formed bystacking two MCC templates. Process II: Removing the DCC template, giving rise to the formation of Ag brochosomes. (b) SEM image of anMCC template. (c) SEM image of a DCC template. Inset: enlarged observation. (d) SEM image of Ag brochosomes with broadband andomnidirectional SERS performance.

Figure 2. Ag brochosomes prepared using different DCC templatesfor different electrodeposition times. (a−c) 200 nm/2 μm DCCtemplate for 30, 60, and 90 s, respectively. (d−f) 350 nm/2 μm for 30,60, and 90 s, respectively. (g−i) 500 nm/2 μm for 30, 60, and 90 s,respectively. Inset in panel h: titled-view image of Ag brochosomes.Scale bar: 1 μm.

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deposition time to achieve the best SERS performance (FigureS2). The size of the pits on the Ag brochosomes has aprominent influence on the SERS performance of the Agbrochosomes. Ag brochosomes prepared using the 500 nm/2μm DCC templates demonstrated the best SERS performance(Figure 3b). Strong SERS signals were observed from the Agbrochosomes even when the R6G concentration was atnanomolar level (Figure 3c). The SERS enhancement factor(EF) of R6G molecules on Ag brochosomes was evaluatedusing the following equation:32

I I N NEF ( / ) ( / )SERS bulk bulk SERS= ×

where ISERS and Ibulk represent the SERS intensity of 10 μL ofR6G aqueous solutions at a concentation of 1 nM dispersed onthe Ag brochosomes over an area of 12 mm2 and the Ramanintensity of 10 μL of R6G aqueous solutions at a concentrationof 10 mM dispersed over a piece of silicon wafer with an areaof 10 mm2, respectively; NSERS and Nbulk represent the numberof R6G molecules exposed to the laser on the Ag brochsomesand the silicon wafer during the SERS measurement. The EFwas estimated to be ∼2.4 × 108 using the 611 cm−1 Ramanpeak of the R6G molecule (Figure S3).The great SERS performance of the Ag brochosomes arises

from the fact that the Ag brochosomes are formed by Agnanoparticles with a mean size of about 60 nm (Figure S4).These Ag nanoparticles are closely packed into the Agbrochosomes with thousands of nanoscale gaps. Thesenanoscale gaps can behave as ultrasensitive SERS sites(known as “hot spots”), giving rise to the outstanding SERSperformance. In contrast, the Ag nanoshells fabricated by

electrodeposition using MCC templates composed of 2 μmsized PS spheres are formed by densely packed tiny Agnanoparticles (see Figure S5a).A linear relationship was observed between the SERS

intensity and the concentration of R6G aqueous solutions inthe range of 10 μM to 1 nM, which could be described by theequation: log C = 5.56 log I − 29.22 (Figure 3d). This meansthat the Ag brochosomes can be used in quantitative SERSanalysis. The reproducibility of the SERS signals over the Agbrochosomes was evaluated by SERS mapping measurementsover an area of 132 μm2 at a scanning step of 1 μm (Figure3e). The uniform color proved the high reproducibility of theSERS signals over the Ag brochosomes. To quantitativelyevaluate the reproducibility of the SERS signals, the intensityof the 611 cm−1 SERS peak from 40 randomly chosen SERSspectra is shown in Figure 3f. The standard deviation wasdetermined to be ∼7.8%, representing a good SERS signalreproducibility. The good SERS signal reproducibility wasattributed to the uniform structure of the Ag brochosomes (seeFigure S1).Surprisingly, strong SERS signals were observed from the Ag

brochosomes prepared using the 500 nm/2 μm DCC template(Figure 2h) under 532, 633, 785, and 1064 nm laser excitation(Figure 4a−d). In contrast, the Ag nanoshells prepared byelectrodeposition on the MCC template composed of 2 μmsized PS spheres (see Figure S5a) and the Ag nanoparticlesprepared by a wet chemical method33 deposited on the Au film(Figure S5b) exhibited obvious SERS signals only under 532and 633 nm laser excitation, while no SERS signals wereobserved under the 785 and 1064 nm laser irradiation. The

Figure 3. SERS performance of the Ag brochosomes. (a) SERS spectra of 100 nM R6G dispersed on the Ag brochosomes prepared using the 500nm/2 μm DCC template after electrodeposition for 30, 60, or 90 s. (b) The SERS spectra of 100 nM R6G dispersed on the Ag nanoshells preparedusing the MCC template composed of 2 μm PS spheres, the 200 nm/2 μm, 350 nm/2 μm, or 500 nm/2 μm DCC template. The electrodepositiontime is 60 s. (c) SERS spectra of R6G dispersed on the Ag brochosomes prepared using the 500 nm/2 μm DCC template after electrodepositionfor 60 s at different concentrations. (d) The linear relationship between the concentration of the R6G molecules and the SERS intensity. The errorbars were obtained based on four independent measurements. (e) SERS mapping result of the 611 cm−1 SERS peak from the R6G moleculesdispersed on the Ag brochosomes. (f) The intensity of the 611 cm−1 SERS peak from 40 randomly chosen SERS spectra.

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thermally evaporated thin Au film (Figure S5c) showed onlyvery weak SERS signals under 532 nm laser irradiation. Noobservable SERS signals were detected under 633, 785, and1064 nm laser excitation. Finite-difference time-domain(FDTD) simulation results proved that strong electromagneticfields existed within the pits of the Ag brochosomes whenexcited by 532, 633, 785, and 1064 nm laser irradiation(Figures 4e−h and S6). This is the reason why the Agbrochosomes displayed good SERS performance under 532,633, 785, and 1064 nm laser excitation.Extremely low reflections (<2%) were observed from the Ag

brochosomes in the visible wavelength range. Even in the near-infrared wavelength range, the reflection is still <10% in abroad incidence angle range (Figure 4i). This means that thelight within this wavelength range can be efficiently absorbed

by the Ag brochosomes, generating strong SERS signals. Thereflection was varied not too much as the direction of theincident light was varied, induced by the spherical morphologyof the Ag brochosomes. This encouraged us to study the angle-dependence of the SERS performance of the Ag brochosomesutilizing a hand-held Raman spectrometer (Figure 4j). Asanticipated, no obvious SERS intensity changes were observedfrom 100 nM R6G stained brochosomes as the incident angle(θ) changed (Figure 4k,l). In contrast, the SERS intensitydecreased rapidly as the incident angle of the laser increasedover the Ag nanoparticle film (Figure S7). This angle-independent SERS performance of the Ag brochosomes isimportant for in-the-field SERS analysis using a hand-heldRaman spectrometer which is difficult to accurately placenormal to the SERS substrate. The omnidirectional SERS

Figure 4. Broadband and omnidirectional SERS performance. (a−d) SERS spectra of R6G molecules dispersed on Ag brochosomes, Ag nanoshells,Ag nanoparticles deposited on Au film, and thermally evaporated Au film under 532, 633, 785, and 1064 nm laser excitation, respectively. Theconcentration of R6G was 100 nM when 532, 633, or 785 nm laser was used, while 10 μM was chosen under 1064 nm laser excitation. (e−h)Electromagnetic field distribution over the Ag brochosomes under 532, 633, 785, and 1064 nm laser irradiation, respectively. (i) Reflection spectraof the Ag brochosomes at different incident angles. (j) Photo of a hand-held Raman spectrometer used for the SERS measurement. Inset: the Agbrochosome SERS substrate. (k) SERS spectra of R6G molecules dispersed on the Ag brochosomes at different incident angles of the laser beam.(l) The intensity of the 611 and 1363 cm−1 SERS peaks at different incident angles. The error bars were obtained based on four independentmeasurements.

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performance was a benefit of the spherical morphology of theAg brochosomes.The Ag brochosomes demonstrated broadband and

omnidirectional SERS performance, indicating promisingapplications in diverse fields. We present SERS detection ofStaphylococcus aureus (S. aureus) bacteria using the Agbrochosomes excited by lasers with various wavelengths(Figure 5a). Strong SERS signals of S. aureus bacteria (2.2 ×106 cfu/mL) were observed on Ag brochosomes under 633and 785 nm laser excitations. No SERS signals were observedunder 532 nm laser excitation, which might be caused by thescreening of the laser caused by bacteria. Notably, the size of S.aureus bacteria is about 700 nm (see inset in Figure 5b andFigure S8), in accordance with the size of the pits on the Agbrochosomes. Therefore, when a S. aureus bacterium is locatedwithin a pit, the interface between the bacteria and the pit ismuch larger than that between the bacteria and traditional flatSERS substrates (e.g., thermally evaporated Au film). There-fore, more area of the S. aureus bacteria can contribute to theSERS signals, similar to the volume-enhanced Ramanscattering (VERS) concept.34 As expected, strong SERS signalsof S. aureus were observed at a concentration of 10 cfu/mLunder 633 nm laser excitation (Figure 5b). The origin of thetypical Raman peaks at 563, 653, 733, 1325, and 1450 cm−1 issummarized in Table S1 in the Supporting Information. TheRaman peak at 1004 cm−1 comes from the SERS substrate(marked by an asterisk in Figure 5b). In contrast, no signals

were observed from the thermally evaporated Au film (Figure5c).To quantitatively evaluate the signal reproducibility during

detecting S. aureus bacteria over the Ag brochosomes, weconducted SERS mapping measurements (Figure 5d). TheSERS signals of S. aureus bacteria dispersed on the Agbrochosomes exhibited high reproducibility and reliability,manifested by the uniform color over the SERS mapping area(Figure 5d). In contrast, the black color of the SERS mappingimage over the thermally evaporated Au film showed theirpoor SERS sensitivity regarding the detection of S. aureusbacteria (Figure 5e). Actually, considering the small sizedifference between the S. aureus bacteria (∼700 nm) and thelaser spot size (∼1 μm) used for SERS measurements, only asingle (or at most, several) S. aureus bacteria can contribute tothe SERS signals regardless of the concentration of S. aureus.This is the reason why the SERS signals varied only slightly asthe concentration of S. aureus increased (Figure 5b). Thisimplies that single S. aureus bacteria could be detected usingthe Ag brochosome SERS substrate. In addition, the locationof S. aureus bacteria over the Ag brochosomes could beaddressed in the SERS mapping image (Figure S9).Furthermore, 35 SERS spectra were randomly chosen fromthe SERS mapping results shown in Figure 5d (Figure S10).The standard deviation of the intensity of the SERS peak at733 cm−1 is only 7.7% (Figure S10), exhibiting a good SERSsignal reproducibility. Importantly, the effective SERSdetection of S. aureus bacteria (2.2 × 108 cfu/mL) on the

Figure 5. SERS detection of S. aureus bacteria using the Ag brochosomes. (a) SERS detection of S. aureus bacteria (2.2 × 106 cfu/mL) usingdifferent lasers. (b) SERS spectra of the S. aureus bacteria at different concentrations under the 633 nm laser excitation. Inset: SEM image of S.aureus bacteria. (c) SERS spectra of S. aureus bacteria (2.2 × 106 cfu/mL) dispersed on the Ag brochosomes and the thermally evaporated Au film.The SERS mapping results of the S. aureus bacteria (2.2 × 106 cfu/mL) dispersed on the Ag brochosomes (d) and the thermally evaporated Au film(e). (f) SERS spectra of S. aureus bacteria (2.2 × 108 cfu/mL) on the Ag brochosomes using a benchtop and hand-held Raman spectrometer.

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Ag brochosomes was further realized using a hand-held Ramanspectrometer (Figure 5f). The SERS performance when usingthe hand-held Raman spectrometer is comparable to the casewhen using the benchtop Raman spectrometer.Further, the broadband SERS performance of Ag brocho-

somes was exhibited when detecting bovine hemoglobinprotein. Obvious SERS signals of bovine hemoglobin proteinwere collected under three lasers, which are in accordance withthe Raman spectrum from the bovine hemoglobin proteinpowder (Figure 6a). Considering that biological species tendto be destroyed during SERS measurements using the shortwavelength lasers, the 785 nm laser is preferred for biologicalSERS detection. As presented in Figure 6b, strong SERSsignals were observed from the bovine hemoglobin protein atconcentrations ranging from 10 000 to 1 μg/mL dispersed onthe Ag brochosomes. The origin of the SERS peaks located at748, 1129, 1172, 1346, and 1630 cm−1 was summarized inTable S2 in the Supporting Information. Moreover, the abilityto perform quantitative analysis of bovine hemoglobin proteinusing the Ag brochosomes was achieved, which could bedescribed by the equation: log C = 4.55 log I − 14.95 (Figure6c). Similarly, SERS mapping measurements were applied toevaluate the uniformity of the SERS performance of the Agbrochosomes regarding detection of bovine hemoglobinprotein. Two-dimensional (2D) SERS mapping results with acolor-coded Raman intensity were drawn based on 42 spectra

with a 1 μm step movement (Figure 6d). All SERS spectrashowed the obvious characteristic SERS peaks of bovinehemoglobin protein. The 10 randomly chosen SERS spectrawere almost identical, manifesting the high reliability of the Agbrochosomes with respect to the detection of bovinehemoglobin protein (Figure 6e). Moreover, the standarddeviation of the Raman intensity at 748 cm−1 was only 3.2%(Figure S11), revealing an outstanding detection reliability.Additionally, obvious SERS signals of bovine hemoglobinprotein (10 μg/mL) were obtained using a hand-held Ramanspectrometer (Figure 6f).In summary, we revealed that the bioinspired Ag

brochosomes have outstanding broadband and omnidirectionalSERS performance, encouraged by the omnidirectionalantireflective properties of the synthetic Ag brochosomes.The uniform structure of the Ag brochosomes induced anextremely high reproducibility of the SERS signals over thewhole SERS substrate with an intensity deviation of <10%. TheAg brochosomes capable of working under a laser in a broadwavelength range overcome the drawback that specificstructures have to be carefully engineered for a certain laserwavelength. This versatility significantly lowered the costtoward engineering SERS substrates for practical applicationsin diverse fields. Furthermore, it is difficult to engineer SERSsubstrates with high SERS enhancement under near-infraredlaser excitation using existing techniques. However, near-

Figure 6. SERS detection of bovine hemoglobin protein on the Ag brochosomes. (a) SERS detection of bovine hemoglobin protein dispersed onthe Ag brochosomes using three lasers and the Raman spectrum of the powder. (b) Concentration-dependent SERS spectra of bovine hemoglobinprotein on Ag brochosomes under 785 nm laser excitation. (c) Relationship between the concentration of the bovine hemoglobin protein and theSERS intensity at 748 cm−1. (d) 2D SERS mapping result of the bovine hemoglobin protein (10 μg/mL). (e) Ten randomly chosen SERS spectraof bovine hemoglobin protein from the SERS mapping image in panel d. (f) SERS spectra of bovine hemoglobin protein (10 μg/mL) on the Agbrochosomes using a benchtop and a hand-held Raman spectrometer.

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infrared wavelength is preferred in biological sensing, becauseit induces negligible influence on the biological species and canpenetrate deeper in tissue for in vivo biosensing. The Agbrochosomes showed outstanding SERS performance undernear-infrared laser excitation, which makes them promising forapplications in biosensing fields. As examples, sensitivedetection of Staphylococcus aureus bacteria and bovinehemoglobin protein were achieved using the Ag brochosomesunder red/near-infrared laser excitation. Additionally, theSERS performance of the Ag brochosomes has a strongtolerance to the incident angle of the laser. This enables the Agbrochosomes to be used under a hand-held Ramanspectrometer which is difficult to place accurately normal tothe SERS substrates. Experimentally, good SERS performancewas observed from the Ag brochosomes under a hand-heldRaman spectrometer at different laser light incident angles.Overall, we have demonstrated that the Ag brochosomes havegood broadband and omnidirectional SERS performance,which are promising for applications in biosensing areas andfor in-the-field sensing using a hand-held Raman spectrometer.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jp-clett.9b02380.

Experimental section, FDTD simulation details, andadditional results (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: [email protected].*E-mail: [email protected].*E-mail: [email protected] Li: 0000-0002-2289-0222Min Zhou: 0000-0002-7319-9570Shikuan Yang: 0000-0001-6662-3057NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe acknowledge funding support by the Zhejiang ProvincialNatural Science Foundation of China (LR19E010001),National Key Research and Development Program of China(2018YFB0703803), and the National Science Foundation ofChina (51702283 and 51871246). M.L. acknowledges thefinancial support from Innovation-Driven Project of CentralSouth University (2018CX002) and Hunan Provincial Science& Technology Program (2017XK2027). The authors acknowl-edge Shanghai Oceanhood Co., Ltd. for providing a hand-heldRaman spectrometer (EVA3000PLUS) for SERS detection.

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