Plasmonic locator with sub‐diffraction‐limited resolution for continuously accurate positioning

Abstract

Both the accurate distance measurement and positioning information at the nanoscale are important for the analysis of micro/nano interactions. Plasmon ruler has been an indispensable optical tool to detect the chemical and biological dynamic processes via distance‐dependent plasmon coupling in the nearly aggregated state. But it cannot disclose the detailed and accurate information of positions and dynamic movements of its two plasmonic components owing to the inherent diffraction limit. Herein, a plasmonic locator is presented which consists of a heterotypic pair of red/blue plasmonic components with significant wavelength difference (∼150 nm). Attributed to the detuned energy (∼0.64 eV) of the two components, the plasmonic locator has the ability of sub‐diffraction‐limited resolution (center to center, 30 nm) and accurate positioning under conventional dark‐field microscopy, making the relative dynamic information of nearby red/blue components be recorded accurately at video rate. As an important complement to the current aggregate science and technology of plasmonics, this newly developed plasmonic locator presents a facile means to realize super‐resolution imaging, accurate positioning, and continuous tracing in chemical and biological interactions at the single‐molecule level.

INTRODUCTION

Both precise distance measurement and accurate positioning at the nanoscale are important parameters in recognizing sundry biological and chemical interactions at the single-molecule level. However, owing to the inherent optical diffraction limit, these two parameters are difficult to be obtained with simple far-field optical imaging techniques. According to the theory of the aggregate science, an aggregate often displays modified properties compared to the components,^[] such as the aggregate of the plasmonic nanoparticles always leads to strong coupling,^[] and the aggregate of AIEgens lead to strong luminescence and new functions.^[] As one of the great advances in plasmonics, plasmon ruler, which is developed to investigate micro/nano interactions through nanoscale distance-dependent plasmon coupling of a homotypic pair of plasmonic nanoparticles,^[] has become a powerful optical tool for exploring conformational dynamics of a single protein,^[] localized mechanical force transduction,^[] and the interactive biophysics and biochemistry between two components, such as complementary DNA sequence, ^[] enzyme-substrate,^[] and chemical reaction substrates.^[] Different from the scattering imaging of a single plasmonic nanoprobe, which reveals the ensemble of reaction,^[] interaction,^[], and connection processes^[] occurred on the surface of the probe, the plasmon ruler enables the recognition of molecular binding^[] and chemical reaction events^[] at the single-molecule level. However, under the dark-field microscopy (DFM), the scattering image of the plasmon ruler only presents as one spot even if it is actually composed of a homotypic pair of plasmonic nanoparticles. As a result, the plasmon ruler is mainly used for some definite processes, such as hybridization/melting of DNA and conformational transformation of the enzyme, but the positioning and dynamic movement information of the two individual components at close range in plasmon ruler, which is also an important parameter for tracing the interaction site and detailed process of the substrates, is not available.

Although great attempts to improve the performance of scattering imaging have been made in full swing recently,^[] only a few are suitable for sub-diffraction-limited resolution and accurate positioning. Holding the advantages of linear polarization modulation, the accurate localization information of nearby anisotropic nanoparticles can be obtained,^[] and a more interesting report is the 3D localization available with integrated light sheet super-resolution microscopy.^[] Compared with some emerging near-field technologies with more demanding requirements, for example, the photoinduced force microscopy^[] and the nanofibre optic force transducers,^[] these methodologies maintain the simplicity and versatility of far-field optical systems. Nevertheless, both anisotropic optical probes and modified imaging modes, such as the angle and lighting modulation, are needed. As a result, these methods have to meet the challenge of much more investigations in terms of applicability. So direct sub-diffraction-limited imaging methods with accurate positioning ability based on common DFM, are still needed but challenging.

To address the aforementioned need, herein the plasmonic locator, which consists of a heterotypic pair of plasmonic nanoparticles with significantly different localized surface plasmon resonance (LSPR) wavelength (red and blue wavelength bands) and detuned energy, is presented. The notable wavelength difference and detuned energy perform a low degree of coupling,^[] so the plasmonic locator presents as highly recognizable non-concentric red/blue spots under DFM, making the accurate individual positioning of two nearby heterotypic plasmonic nanoparticles can be achieved easily (Scheme ). The diffusion of a single plasmonic nanoparticle can be traced effectively,^[] therefore, by combing the high-resolution positioning ability of the plasmonic locator, the dynamic process and the relative movement between the dual-color components within the sub-diffraction-limited distances could also be investigated and recorded. In addition, the simple far-field and non-scanning imaging mode of DFM is able to guarantee high temporal resolution and to further increase their broad impact.

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RESULTS AND DISCUSSION

As a proof-of-principle, we experimentally selected a plasmonic locator which consists of a pair of nearby sliver nanorod (AgNR) and silver nanosphere (AgNS) to explore the sub-diffraction-limited resolution and accurate positioning. As known, the resolution of far-field optical microscopy is bound to >200 nm at the visible wavelength range.^[] Silver nanoparticles (AgNPs) containing mainly blue silver nanospheres (AgNSs) and a few red sliver nanorods (AgNRs) (Figure ) were chosen to investigate the super-resolution capability. The co-localized images by DFM and scanning electron microscopy (SEM) revealed that the red/blue spots under DFM were attributed to the AgNR and AgNS (Figure ), respectively. Under DFM, although the edges of the two spots, especially for the two blue ones, were overlapping and fused, both the same blue-blue and red-blue dual-color nanoparticles at a large distance (∼500 nm) could be confirmed to be distinguished easily (Figure ). And in a shorter distance (∼350 nm), the imaging spot of two blue AgNSs began to exhibit obviously decreased resolution (Figure ). Furthermore, neither two blue AgNSs nor two green AuNSs at a short distance (<200 nm) could be discriminated (Figure 1A-D, Figure ). They were all shown as one fused spot, as a result, the accurate positioning of the individual plasmonic components couldn't be available without the confirmation of the SEM imaging results. Besides, under DFM, the coupling of two blue AgNSs at short distances (<100 nm) showed a red-shift from blue to cyan, which was indeed confused to the imaging results of a short ellipsoidal AgNP (Figure 1A, Figure ). So, under the commonly used far-field DFM, the accurate positioning of two nearby plasmonic probes, including the plasmon ruler, at a sub-diffraction-limited distance was a tricky problem.

Although the two closely nearby (∼30 nm, center to center) red AgNR (λ_max, 620 nm) and blue AgNS (λ_max, 470 nm) were presented as merged spots, the significant detuned energy (∼0.64 eV) and color difference ensured the recognition of their individual accurate positions. The center of AgNS was in the southwest direction (∼10 degrees) of the AuNR (Figure ) from directly DFM results, which was consistent with the actual situation confirmed by the SEM imaging result. The two peaks in the red and blue wavelength bands in the single nanoparticle spectrum were also consistent with the red-blue spots very well, revealing that the existence of the obvious blue color in the variegated spots was attributed to the AgNS (Figure ). Between the nearby plasmonic nanoparticles, there was obvious coupling, and the spectra always exhibited noticeable red-shift and intensity enhancement.^[] For nearby AgNSs, the scattering wavelength is usually red-shifted from blue to cyan, no matter in the previous report^[] or the experiment results (Figure ; Figures , , and ). Herein, the nearby red AgNS did not lead to the classical cyan, suggesting the weak coupling in the condition of detuned energy should be important for the super-resolution. Besides, by comparing the RGB line distribution of the plasmonic locator and a single red AuNR, the towering blue signal and its deviation from the center of the red spot also indicated the existence of the blue AgNS (Figure ). Therefore, the plasmonic locator successfully achieved the sub-diffraction-limited resolution and positioning.

Image reconstruction was used to investigate the imaging accuracy and the sub-diffraction-limited resolution of the plasmonic locator. Different from the direct DFM imaging with the white light source (Figure ), the monochromatic images of the two components of the plasmonic locator obtained by adding monochrome filters with the matched energy were used to restructure the image (Figure ). LSPR of plasmonic nanoparticles could only be stimulated by incident light with matched energy, otherwise, it would be in silence and contribute little to the nearby particles. So the information of positions obtained from the reconstructed image could reflect the actual locations of the individual particles. By comparing with the restructured image, the direct DFM imaging could also define the accurate location information of the plasmonic locator (Figure ). Therefore, the plasmonic locator was confirmed to be an efficient tool for sub-diffraction-limited resolution and positioning without additional wavelength modulation procedure, which is time consuming.

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The scattering intensity between the components of the plasmonic locator sometimes had a significant difference, and one particle might form only a slight spot which was hard to be distinguished. By extending the exposure time, a plasmonic locator containing a slight blue one could be defined slightly better under DFM (Figure ). And by using a condenser with a narrowed annulus (Figure ), the imaging resolution could be increased weakly for both the monochromatic and dual-color nearby nanoparticles (Figures and ), and more importantly, the scattering intensity difference between the particles could be reduced. Simultaneously, the red/blue plasmonic locator could be confirmed easily, avoiding excessive exposure (Figure ). But the rational design of a plasmonic locator with similar scattering intensities would be a wise choice.

Besides the above static imaging resolution and positioning research, the accurate location and dynamic movement recording of the plasmonic locator were also investigated. Isotropic gold nanoshells (AuNSHs) were selected as the red particle because its combination with the blue AgNS could be used to further confirm that the sub-diffraction-limited resolution and positioning capability of the plasmonic locator were independent of the anisotropy (Figure ). The following results demonstrated that high resolution could also be achieved effectively. The slight relative movement between a pair of nearby red/blue nanoparticles could be captured from the blue spot leaned out of the red spot mildly (Figure ; Movie ). The obvious relative movement of red-blue nanoparticles, which displayed a more complex trajectory than two faraway red-blue nanoparticles (Movie ), could be recorded detailed, including several coincidence processes (Figure ; Movie ). During the separation process of two nearby blue AgNSs, the separation could only be confirmed when the separated distance was large enough to form a slender spot (image at the 2 s in Figure , Movie ). Obviously, the plasmonic locater could supply more dynamic information than the monotonous recording of connection/disconnection and conformational transformation processes. In addition, the plasmonic locator could also be used for the positioning of the complex combination of nanoparticles (Figure ), and the resolution between the blue and red ones was undisturbed from the low resolution of the blue-blue and red-red pairs.

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CONCLUSION

In summary, our results experimentally confirm the sub-diffraction-limited resolution and positioning capability of the plasmonic locator by conventional DFM without wavelength modulation. The plasmonic locator is an important supplement to the existing and widely used plasmon ruler specialized in fine distance measurement. Combining the flourishing technologies for functionalization and assembly of plasmonic nanoparticles, a plasmonic locator would tremendously expand the field of application in the exploration and accurate analysis of the chemical and biological reaction mechanism and dynamic interaction process.

EXPERIMENTAL SECTION

Materials: The following reagents were all commercially available and used without further purification. AgNO₃ was purchased from Shenbo Reagents Ltd. (Shanghai, China, 99.8%). HAuCl₄ was from Sinopharm Reagents Ltd. (Shanghai, China, Gold content 47.8%). Sodium citrate was obtained from Ruijinte Reagents Ltd. (Tianjin, China, 99.0%). All solutions were prepared using ultra-pure water (Millipore, 18.2 MΩ). The gold nanoshells (AuNSHs) were purchased from Jicang Nano Technology Co., Ltd (Nanjing, China).

Apparatus: Scanning electron microscopy (SEM) was carried out on an S-4800 scanning electron microscope (Hitachi, Japan). Extinction spectra of AgNPs, AuNPs, and AuNSHs were measured with a U-3010 spectrophotometer (Hitachi, Japan). Dark-field microscopy (DFM) images were obtained by a BX51 optical microscope (Olympus, Japan). The equipped charge-coupled device (CCD) camera was a DP72 single-chip true-color CCD (Olympus, Japan). The used light source was a U-LH100-3 halogen source (100 W). A condenser (U-DCW, 1.2–1.4) and a 100× object lens were used. The single nanoparticle scattering spectra were determined by a spectrograph (MicroSpec-2300i, Roper Scientific) and the intensified CCD camera (PI-MAX, Princeton Instrument). During the scattering, the cedar oil with a refractive index of ∼1.5 was used.

Preparation of AgNPs: The used silver nanoparticles (AgNPs) were prepared by the following procedures. Briefly, 50 mL glycerol/water mixture (40 vol%) was stirred violently and was heated up to 100°C. 8.5 mg AgNO₃ was added, and after about 1 min, 0.8 mL 3% sodium citrate (wt%) was added. Keep stirring for 15 min to finish the aging process, and then, larger AgNPs about 50.0 nm were obtained by adding the AgNO₃ (100 μL) and sodium citrate (150 μL) for about 3–5 times alternately for a regrowth procedure.

Preparation of AuNPs: The used gold nanoparticles (AuNPs) were prepared by similar procedures as that of the AgNPs with some modifications. To 50 mL of boiling water containing 1 mL (1%, wt%) HAuCl₄, 200 μL of 3% sodium citrate (wt%) was added. Keep stirring for 15 min to finish the aging process, and then, larger AuNPs about 50.0 nm were obtained by adding the HAuCl₄ (100 μL) and sodium citrate (150 μL) about 5 times alternately for a regrowth procedure.

Co-localization DFM and SEM imaging: Firstly, the nanoparticles were deposited on the ITO conductive glass, on which cross scratches were added to the middle section before. Then the particles were imaged under the DFM, and the places containing the nearby dual-color or same colored nanoparticles were captured. Herein, the position of this place should be confirmed with the help of the cross scratches. The SEM image of this place was obtained. The co-localization images of the same particles were then used to analyze the positioning and optical information.

Monochromatic imaging for image reconstruction: The monochromatic imaging of the nearby red-blue nanoparticles was achieved by adding red and blue filters between the white light source and the dark-field condenser. Because the filters induced the reduction of the incident light irradiated to the samples, the different exposure time was used to obtain the images with suitable scattering intensity. The obtained monochromatic images of the same place were used to reconstruct the fusion image.

Imaging by condenser with narrowed annulus: The direct DFM imaging is by the normal dark-filed condenser. The diameter of the black light stop in the condenser was defined. Circle stops with different diameters slightly larger than the original black light stop were prepared. Then the stops were added into the condenser accurately concentric with the original ones to make sure the uniformity of the light source. The narrowed annulus also led to the reduced incident light intensity, different exposure time was used to obtain the images with suitable scattering intensity.

Imaging of AgNS and AuNSH with relative movement: Firstly, the AgNPs were deposited on the common glass slides which had a weak negative charge on the surface. Then the excess undeposited nanoparticles were washed away. Diluted AuNSHs were next added to the glass groove to avoid too many particles causing confusion in imaging vision. The vision containing a pair of nearby red and blue spot was selected for monitoring. Continuous recording mode was used to record the whole dynamic process of the relative movement of the dual-color nanoprobes. Imaging of two nearby blue AgNPs with relative movement was through a similar operation.

ACKNOWLEDGMENTS

This work was financially supported by the National Natural Science Foundation of China (NSFC, Nos. 22134005, 21874109, and 21605125) and the Natural Science Foundation of Chongqing (No. cstc2020jcyj-msxmX0992).

CONFLICT OF INTEREST

The authors declare that there is no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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