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Plasmon-enhanced ultra-sensitive surface analysis using Ag nanoantenna Chao-Yu Li, Jinhong Gao, Jun Yi, Xia-Guang Zhang, Xiao-Dan Cao, Meng Meng, Chen Wang, Ya-Ping Huang, San-Jun Zhang, De-Yin Wu, Chuan-Liu Wu, Jian-Hua Xu, Zhong-Qun Tian, and Jian-Feng Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04113 • Publication Date (Web): 25 Dec 2017 Downloaded from http://pubs.acs.org on December 27, 2017

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Analytical Chemistry

Manuscript:

Plasmon-enhanced ultra-sensitive surface analysis using Ag nanoantenna Chao-Yu Li,†,‡ Jin-Hong Gao,†,‡ Jun Yi,†,‡ Xia-Guang Zhang,† Xiao-Dan Cao,§ Meng Meng,† Chen Wang,† Ya-Ping Huang,† San-Jun Zhang,§ De-Yin Wu,† Chuan-Liu Wu,†,* Jian-Hua Xu,§ ZhongQun Tian,† and Jian-Feng Li†,* †

MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. §

State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, Shanghai 200062, China.

*

Email of corresponding author: [email protected] (+86-592-2186192), [email protected] (+86-592-2183206)

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Plasmon-enhanced ultra-sensitive surface analysis using Ag nanoantenna Chao-Yu Li,†,‡ Jin-Hong Gao,†,‡ Jun Yi,†,‡ Xia-Guang Zhang,† Xiao-Dan Cao,§ Meng Meng,† Chen Wang,† Ya-Ping Huang,† San-Jun Zhang,§ De-Yin Wu,† Chuan-Liu Wu,†,* Jian-Hua Xu,§ ZhongQun Tian,† and Jian-Feng Li†,* †

MOE Key Laboratory of Spectrochemical Analysis and Instrumentation, State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China. §

State Key Laboratory of Precision Spectroscopy, Department of Physics, East China Normal University, Shanghai 200062, China.

ABSTRACT: Raman scattering and fluorescence spectroscopy permeate analytic science and are featured in the family of plasmon-enhanced spectroscopy (PES). However, the modest enhancement of plasmon-enhanced fluorescence (PEF) significantly limits the sensitivity in surface analysis and material characterization. Herein, we report a Ag nanoantenna platform, which fulfills simultaneous ultra-strong emission (an optimum average enhancement of 105-fold) and ultra-fast emission rate (~280-fold) in PES. For the application in surface science, this platform has been examined with diverse fluorophores. Meanwhile, we utilize finite-element method (FEM) and time-dependent density functional theory (TDDFT) to comprehensively investigate the mechanism of largely enhanced radiative decay. PES with shell-isolated Ag nanoantenna will open a wealth of advanced scenario for ultra-sensitive surface analysis.

Owing to their unique access to the nanometer realm, advances in the field of PES has dramatically expanded our knowledge of various fields, including surface science, biological analysis, and material science.1-3 Especially, the single-molecule fluorescence spectroscopy is widely applied for biological imaging with nanoscale reolustion.46 With the use of metal nanostructures, light fields can be confined to the sub-wavelength region owing to the excitation of a collective electronic resonance, which is known as surface plasmon resonance (SPR).3, 7-10 When the fluorophore is placed near the metal surface, the excitation efficiency, spontaneous emission rate, and photostability will be modified due to interaction with plasmon, which has been considerably investigated by PEF.11-19 To increase the photon collection efficiency, the metal nanostructure can also act as a nanoantenna for the controllable emission by coupling the radiation of emitter with its directional far-field scattering.20-21 Meanwhile, the electromagnetic local density of states (LDOS) is maximized around the SPR spectral region due to excitation of localized surface plsmon, which enhances the radiative and nonradiative decay rates of excited molecule.22 Although PEF is inspired and promoted soon after the discovery of surface-enhanced Raman scattering (SERS),7, 23 the enhancement of PEF is much smaller than that of SERS and normally only 10~20-fold, which limits its sensitivity in surface analysis.24-25 Extensive efforts have

been spent on largely enhancing emission intensity, such as the design of plasmonic nanostructure for the fast spontaneous emission rate.25 Because plasmonic nanostrucutre will facilitate the enhancement of local electromagnetic field, a key point deciding the emission intensity of luminescent materials, but the close contact with metal surface will cause the huge fluorescence quenching of dyes.26-28 Although the enhanced spontaneous emission rate is a great reason of the emission intensity enhancement, it is desirable to put forward a significant question that how strong and how fast the emission can achieve simultaneously. So far, it is challenging to fulfill both the largely enhanced emission intensity (a high quantum efficiency) and ultra-fast spontaneous emission rate experimentally.29 Watersoluble metal nanoparticles (NPs), which are simple to prepare and can provide high near-field enhancement with remarkable adaptability on diverse substrates, are highly popular in SERS.30-31 To further motivate the application of PES in both physical chemistry and analytical science, it is highly desirable to develop acosteffective and flexible plasmonic platform fulfilling simultaneous ultra-fast emission rate and ultra-strong emission intensity. Recently, an innovative spectroscopic methodology called shell-isolated nanoparticle-enhanced Raman scattering (SHINERS) has emerged in the field of PES.32-34


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Analytical Chemistry To aim for large enhancement of fluorescence intensity and fast emission rate, we have designed a Ag shellisolated nanoparticles (Ag SHINs)-enhanced fluorescence platform, where the silica shell of the can be readily tuned to aviod the great fluorescence quenching, while providing a largely enhanced local electromagnetic field. Herein, a sub-monolayer of Ag shell-isolated nanoparticles (Ag SHINs) is simply cast onto dye-functionalized substrates to manipulate the communications between plasmon and emitter, and the generality has been examined using various fluorophores covering most of the visible spectrum region. By introducing Ag SHINs, the compact dielectric shell of SHINs optimizes the “dequenching” effect while maintaining a high local filed enhancement. Thus, the ultra-fast radiative emission decay shows an overwhelming dominance in the competition with nonradiative energy loss. Especially, the maximum averaged enhancement is up to ~1.2 × 105-fold. By minimizing the dissipated energy, we have obtained a quantum yield more than 50% with the presence of high emission rate. The enhanced fluorescence intensity would alleviate the harmful effect of high power exciting beam and multi-labeling of biomolecule. In addition, the shorted lifetime of excited state would facilitate more accurate research of biomolecule dynamics by offering elaborate time-resolved fluorescence signal.

EXPERIMENTAL SECTION In the article, different fluorescent probes were modified covalently on Ag-based substrates via amino-related chemistry. Amino-group was functionalized on Ag film or SiO2 spacer based on thiol-metal chemistry or silane reaction strategy separately before fluorescent probes attaching. Briefly, Ag film was immersed in the ethanol solution of thiols (1-amino-1-undecanethiol hydrochloride (AUT) and 1-octylthiol (OT)) with the total concentration of 1 mM. AUT/OT ratio could be varied to regulate the density of amino group; 100% amino density would be expected in 1 mM AUT, while 50% amino density in the ratio of AUT/OT=1/1. In the case of SiO2, an ethanol solution of silane coupling agent (the silane solution is slightly modified with water and acetic acid) was prepared in which the substrates were immersed. Similarly, 50% amino density on SiO2 would be obtained under the ratio of 3aminopropyltriethoxysilane (APTS)/ Triethoxypropylsilane (TEPS)=1/1 while 100% in mere APTS solution. After amino-group functionalization, all the substrates were washed thoroughly with ethanol and stored under vacuum condition before fluorescent probes modification. Fluorescein isothiocyanate (FITC) and rhodamine isothiocyanate (RITC) were functionalized to substrate with isothiocyanate-amino chemistry in slightly basic ethanol solution while BODIPY 650 with NHS-amino chemistry in DMF. The reactions were performed at the concentrations of three probes fixed at 250 μM and kept for 24 h in darkness. After fluorescent probes modification, the all substrates were washed thoroughly with ethanol and stored

under vacuum condition before conducting proceeding experiments. The Ag film substrate was prepared via e-beam evaporation of 200 nm-thickness Ag film onto Si(111) wafer. For Ag + 2/5 nm spacer substrate, 2 or 5 nm dielectric layer was then deposited with atomic layer deposition system (R-200 Advanced), in which tris(dimethylamino)silane (TDMAS), trimethylaluminium (TMA), and ultrapure water were used as silicon precursor, aluminum precursor, and oxidant, respectively. 2 nm SiO2 layer was used as 2 nm spacer directly. For 5 nm spacer, 3 nm Al2O3 was deposited first, and then followed with 2 nm SiO2 layer. Ag SHINs with various dielectric shell thicknesses were prepared according to the shell-isolated method. The 96 ± 10 nm spherical Ag nanoparticles were synthesized with a seed growth method in which 16 nm Au nanoparticles were used as seeds: 38 mM sodium citrate (1.5 mL) was added to 0.24 mM boiling HAuCl4 (50 ml) under vigorous stirring, and then the Au seeds sol were diluted 45 times. Next, the diluted Au seeds were mixed with sodium citrate and AA, and then AgClO4 was added to the mixture slowly. The concentration of AA, AgClO4 and sodium citrate in mixture was 1.84, 1.25 and 1.25 mM, respectively. To obtain the Ag SHINs with various shell thicknesses, as-prepared 96 nm Ag sol was diluted two times with water. And then NaBH4, APTMS, and sodium silicate solution were added to the diluted Ag sol one by one. The amount of APTMS, NaBH4, and sodium silicate in mixture was 0.22 mM, 5.5 mM, and 0.045%, respectively. During the preparation, the pH value of growth solution was modified by H2SO4 to be ~9.7. The mixture was immediately transferred to a 90 °C bath and was stirred for 60 min. Then the bath temperature was tuned down to 60 °C and growth times were changed from 0 to 35, and 150 min, thus the shell thicknesses are varying from 2 to 6, and 10 nm, respectively. To get a silica shell of 20 nm, 4 ml of Ag sol was diluted to 30 ml, while other additions are kept the same except the pH value was tuned to ~7 to facilitate the hydrolysis of silicate. After 4 h heating time, Ag SHINs with 20 nm silica shell was obtained. The asprepared Ag SHINs have been concentrated and then dispersed into ultrapure water again. For fluorescence experiments, the Ag SHINs are drop cast onto the substrate, and then dried at room temperature.

RESULTS AND DISCUSSION With the SHINERS method, the dielectric shell thicknesses were delicately modified from 2 to 6, 10, and 20 nm. The coupling configuration of the sub-monolayer of Ag SHINs-RITC film-Ag substrate is shown in Figure 1A, and is further characterized by the electron microscopy images in Figure S1. Meantime, the DFT is used to simulate the orientation of the transition dipole (Figure 1B), and FEM calculations have been carried out to explore the enhancement mechanism and then distinguish the Ag SHINs-enhanced emission approach (Figure 1C). To explore the emission process and further



reduce the nonradiative energy loss, we optimized the distance between the dyes and the Ag substrate by depositing an ultra-thin dielectric spacer using atomic layer deposition (ALD) technique.

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larger than 103 had been predicted theoretically, the enhancement of spontaneous emission rate more than 200fold is remarkable.21, 35 With Ag SHINs, the presented Purcell factor is the largest in PEF using mere organic dyes with short lifetime of excited state at room temperature.

Figure 1. (A) Schematic diagram of Ag SHINs-enhanced spectroscopy. (B) Calculated optimum molecular conformation of RITC on silica substrate where the blue arrow denotes transition dipole. (C) Simulated radiation patterns in Ag SHINs-enhanced fluorescence. Blue and gray shadows present the patterns with and without Ag SHINs on Ag + 2nm spacer substrate. The collection angle (NA=0.8) is indicated by red area. Three-dimensional radiation patterns with (upper) and without (bottom) Ag SHINs are presented on the right.

Figure 2A, B, and C reveal the Ag SHINs-enhanced fluorescence of RITC on a Ag film deposited with 0-, 2-, and 5-nm dielectric spacer (termed Ag film, Ag + 2-nm spacer substrate, and Ag + 5-nm spacer substrate here after). Figure S2 shows the TEM characterizations of the spacers and the ratio of reference intensities on these three substrates. The fluorescence enhancement is calculated according to the ratio of ISHIN to IRef, where ISHIN and IRef are the background-corrected SHINs-enhanced fluorescence and reference intensities on the corresponding substrate, respectively. All the optimum emission intensities on different Ag-based substrates were obtained substrates were obtained with 2 nm-shell Ag SHINs, and the corresponding enhancements on Ag film, Ag + 2-nm spacer substrate, and Ag + 5-nm spacer substrate are up to ~39,000, ~42,000, and ~3,700-fold, respectively, with respect to reference on the related substrate. The absolute intensity with 2 nm-shell SHINs on the Ag + 2-nm spacer substrate is 2.8 times that on the Ag film, and 3.2 times that on the Ag + 5-nm spacer substrate. The details of the enhancements with different shell thicknesses on Agbased substrates are summarized in Figure 2E. To investigate the Purcell factor of SHINs-enhanced fluorescence, the time-correlated single photon counting (TCSPC) measurement was performed. Figure 2D shows the fluorescence lifetime of RITC on the Ag + 2-nm spacer substrate, and the curve obtained with 2-nm-shell SHINs almost coincides with that of the instrument response function (IRF) which indicates a detector-limited emission decay rate. By reconvoluting IRF, a fitting lifetime of around 10 ps is obtained, and a shorter result is beyond the limits of instrumental detection. Hence, a conservative estimate for the Purcell factor of around 280 with respect to the reference on glass (the average lifetime on glass is ~2,790 ps) is obtained. Although a Purcell factor

Figure 2. Ag SHINs-enhanced emission of RITC on Ag-based substrates. (A to C) Ag SHINs-enhanced emission spectra of RITC on Ag film, Ag + 2-nm spacer substrate, and Ag + 5-nm spacer substrate, respectively. The dielectric shell thicknesses of Ag SHINs are changed from 2 to 6, 10, and 20 nm. All the fluorescence spectra are background corrected and the intensities have been normalized to the value obtained with 2 nm-shell Ag SHINs in (B) (blue curve). All the reference intensities from related substrates have been magnified 200fold for comparison. The insets show the corresponding cartoon diagrams. (D) The Ag SHINs-mediated fluorescence decay measurement of RITC on the Ag + 2-nm spacer substrate. (E) Fluorescence enhancements of RITC on different substrates with respect to the reference on corresponding substrate. Error bars indicate one standard deviation which propagated errors from the references as shown in Figure S2. (F) Simulated shell-thickness-dependent fluorescence EF, excitation EF, Purcell factor, and quantum yield of Ag SHINsenhanced RITC on the Ag + 2-nm spacer substrate. (G to J) Simulated excitation EF, Purcell factor, quantum yield, and fluorescence EF, where the silica shell thickness of Ag SHIN is set at 2 nm.

To elucidate the enhancement mechanism, the optical field and fluorescence enhancements were calculated via FEM method, while the optimum molecular conformation of RITC attached to the substrate surface was calculated via time-dependent density functional theory (TD-DFT). The calculation details are presented in Sup-


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Analytical Chemistry plementary Information Section 5. Figure 2F summarizes the simulated results with different shell thicknesses, in which the fluorescence EF decreases dramatically with increasing shell thickness while the Purcell factor and quantum yield decline slowly. These shell-thicknessdependent results indicate a dominant role of the excitation efficiency. Figure 2G-J reveal the calculated excitation EF, Purcell factor, quantum yield, and fluorescence EF of the optimum coupling configuration (2-nm-shell SHINs coupled with Ag + 2-nm spacer substrate). With the concentration effect of incident light, the excitation efficiency of the molecules nearby is prominently enhanced. Meanwhile, LDOS is maximized around the SPR region and accelerates the spontaneous emission of emitter.22 Through the combination of enhanced absorption and modified quantum yield, a simulated average enhancement up to 40,640-fold is obtained, which is well correlated with the experimental results. Considering the applications in biological system, we have performed the simulations of fluorescence enhancements for Ag SHINs in various environments (e.g. water) as presented in Figure S3.

Figure 3. (A) Single 2-nm-shell Ag SHIN-enhanced spectroscopy of RITC (black curve), scattering spectra (red curve), simulated scattering spectra (pink curve), and reference spectra (without SHIN，blue curve) on Ag +2-nm spacer substrate. (B) Corresponding SEM image, in which the scale bar is 150 nm. Inset shows the corresponding dark-field photograph. (C and D) Corresponding simulated excitation EF and fluorescence EF, respectively. (E to H) Another series of results obtained from a single Ag SHIN with 10-nm shell. All the fluorescence intensities in (A) and (E) have been normalized.

To further investigate the effect of plasmon resonance upon the radiative process, single Ag SHIN-enhanced fluorescence (SSHINEF) of RITC has also been performed on Ag + 2-nm spacer substrate, as shown in Figure 3. Although thinner dielectric shell would facilitate the higher electric field, the enhancement of single Ag SHIN with 2nm shell is modest (the maximum enhancement is ~730fold, Figure 3A) as compared with the value from 10-nm shell (the maximum enhancement is ~2,000-fold, Figure 3E). Because of the coupling between Ag core and Ag film, the SPR scattering from 10-nm-shell SHIN is around 550 nm, which is better matched by the emission peak of RITC. Moreover, the 10-nm shell could further reduce the

considerable energy loss to metal surface. Overall, a higher enhancement is obtained with 10-nm shell in SSHINEF experiments. These experimental results are corroborated by the simulated results in Figure 3C, D, G, and H. More SSHINEF experimental results are supplemented in Figure S4. As indicated by these results, we found the “reshaping” effect of SPR upon the emission spectra, while a higher enhancement is obtained with a better match between SPR and emission band.36-37

Figure 4. Ag SHINs-enhanced spectroscopy of (A) FITC and (E) BODIPY 650 on Ag + 2-nm spacer substrates. Both the reference intensities on the Ag + 2-nm spacer (black curve) are magnified 500 times. (B) and (F) show the excitation and emission spectra of 10 nM FITC and 1 μM BODIPY 650, respectively, in ethanol. (C) and (G) reveal the shell thicknessdependent enhancements of FITC and BODIPY 650 as a function of substrate. Error bars indicate one standard deviation which propagated errors from the references as shown in Figure S2c. (D) and (H) present the corresponding Ag SHINs-mediated fluorescence decay measurements of FITC and BODIPY 650.

Although fluorescence enhancements of more than 103fold have been achieved via fabricated Au nanocavities,28, 35 these top-down preparations are cost-prohibitive and the generalities are limited in the red to near-infrared spectrum region. It is necessary to develop a platform with excellent generality to cover most of the visible spec-



trum while maintaining high enhancement. To illustrate the advantage of this method upon the fluorophorespecific issue, the dye was changed to FITC and BODIPY 650 for 488 and 633 nm excitations (as shown in Figure 4 and S6). As revealed in Figure 4A and E, with a 2-nm-shell Ag SHINs, the enhancements for FITC and BODIPY 650 are ~23,400- and ~109,000-fold, respectively. Especially, the maximum enhancement of up to ~120,000-fold is observed from BODIPY 650 on the Ag film. Similar to the case with RITC, both the emission intensities of FITC and BODIPY 650 on Ag + 2-nm spacer substrates are reduced with increasing shell thickness. As shown in Figure 4D and H, the lifetimes of FITC and BODIPY 650 in the presence of 2-nm shell Ag SHINs are measured to be 20 and 10 ps, respectively. CONCLUSIONS In summary, we have demonstrated an ultra-sensitive and cost-effective surface analysis methodology with excellent generality for diverse fluorophores. Particularly, the dielectric shell-isolated Ag nanoparticles are capable of achieving more than 105-fold fluorescence enhancement and large spontaneous emission rate enhancement. DFT and FEM calculations are performed for the in-depth understanding of radiative emission enhancement mechanism and indicate a dominant role of excitation field in total emission intensity. In our proposed platform, the particle-film coupling structures provide huge enhancements in spontaneous emission rate. More importantly, via rational controlling the shell thickness and spacer thickness, the quenching problem is overcome, which is a common problem is plasmon-enhanced fluorescence. Besides, the directional emission property also enables a high collection efficiency. Therefore, our platform also simultaneously provides a fast and ultra-strong fluorescence emission. The proposed method offers a powerful tool with high emission intensity and significant shorted lifetime of excited state. In addition, the much lower background, due to the fluorescence quench effect of metal substrate, will provide higher signal/noise ratio. Hence, considering higher level of background signals in complicated bio-relevant detection, the SHIN method with intrinsic ultralow background would offer great advantage over other proposed methods.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The supplementary experimental section, single Ag SHINenhanced fluorescence of RITC, calculations details (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected]

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Author Contributions ‡These authors contributed equally.

ACKNOWLEDGMENT We thank Prof. B. Ren, Prof. Z. L. Yang, Dr. S. Chen, and L. Li for discussions. This work was supported by NSFC (21522508, 21475109, and 21521004), "111" Project (B16029 and B17027), Research Funds for Central Universities (20720150039) and Thousand Youth Talents Plan of China.

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