Large field-of-view plasmonic scattering imaging and sensing of nanoparticles with isotropic point-spread-function

Domain: earth_science
Published: 2025-06-02
Editorial outcome: minor_revision

Abstract
Label-free optical imaging and sensing of single nanoparticles are vital for fundamental research, disease diagnosis, and nanomaterial studies. Surface plasmon resonance microscopy (SPRM) is a label-free detection technology which is widely used in the detection of single nanoparticles. However, conventional SPRM suffers from poor spatial resolution, a limited field-of-view, system complexity, and high operating costs. In this study, we introduce a compact, low-cost, and large field-of-view chip-based plasmonic scattering microscopy (Chip-PSM). Compared with SPRM, Chip-PSM retains high detection sensitivity and in situ label-free analysis capability, while offering a larger field-of-view, an isotropic point-spread-function and higher spatial resolution. With these advantages, Chip-PSM enables detecting and imaging dielectric nanoparticles, gold nanoparticles, and biological samples. Additionally, the hygroscopic growth dynamics of aerosol nanoparticles and the chemical reactions occurring on nanocrystals are successfully characterized via Chip-PSM. We anticipate that the proposed Chip-PSM will have broad applications across many scientific fields, including physics, chemistry, and atmospheric sciences.

Introduction
Imaging and detecting single nanoparticles via optical label-free methods is critical for environmental monitoring, understanding chemical reaction mechanisms, and studying biological processes1–7. Surface plasmon resonance microscopy (SPRM) is a popular label-free detection technique because of its extraordinary sensitivity to tiny signals. Surface plasmons are one type of evanescent field and can largely enhance light‒material interactions. Therefore, surface plasmons can be used to develop highly sensitive label-free imaging methods such as SPRM. Recently, SPRM has been employed for imaging single cells8–10, subcellular organelles11, viruses12, nanoparticles13–16, exosomes17, and chemical reactions18,19 owing to its extreme sensitivity. However, SPRM has two significant disadvantages that limit its broader application. First, SPRM suffers from poor spatial resolution because of interference between the reflective wave and the leakage radiation scattered by nanoparticles, resulting in a point-spread-function with a parabolic tail17,20. Although using azimuthal rotation illumination can improve spatial resolution21,22, it requires a pair of high-speed scanning galvanometers and complex optical path adjustments. Second, SPRM images are formed by collecting the reflected light from the gold surface. The reflection light creates a strong background noise on the image, reducing image contrast and making it difficult to image small nanoparticles. Recently, a research group proposed plasmonic scattering microscopy (PSM) to overcome these two disadvantages23–25. Unlike SPRM, PSM collects plasmonic waves scattered by nanoparticles with an air objective placed on top of the sample. PSM retains the advantages of SPRM because surface plasmons still function as the illumination field. At the same time, the scattering light is directly imaged to form PSM images without collecting the reflection light, which results in high image contrast and high spatial resolution.

However, PSM require bulky, complicated, and expensive optical setups, such as a high numerical aperture objective lens used to excite surface plasmons, which are cumbersome to operate, especially for untrained users. Additionally, the angle of incident light at the metal‒dielectric interface must be meticulously adjusted to achieve momentum matching for surface plasmons’ excitation26. Furthermore, the use of a high numerical aperture (NA) oil-immersion objective to excite the surface plasmons in the PSM and SPRM limits the field-of-view of the imaging system. This makes simultaneous imaging and sensing of large quantities of nanoparticles difficult or impossible. In recent years, interest in developing chip-based imaging instruments to simplify the optical setup and expand the field-of-view has increased27–29.

In this work, we propose a compact, low-cost, and large field-of-view chip-based plasmonic scattering microscopy (Chip-PSM) for the label-free imaging and sensing of nanoparticles with isotropic point-spread-function. Chip-PSM greatly simplifies the PSM setup by using a compact optical illumination device to excite surface plasmons. This compact optical illumination device is a combination of a gold-coated glass slide, a well-designed one-dimensional photonic crystal (1DPC), a polymer layer and an LED light source. All these components are of planar shape and easily integrated. This compact optical illumination device is much simpler than the devices used in the previous works on chip-based label-free optical microscopy28,29, thus it enables significant miniaturization and simple operation of the microscope setup. This compact optical illumination device can be installed in a conventional optical microscopy to realize plasmonic imaging. Omnidirectional surface plasmons illumination can be excited via this compact optical illumination device; thus, the Chip-PSM will provide an isotropic point-spread-function, which cannot be realized via conventional PSM or SPRM. Like PSM or evanescent scattering microscopy30, Chip-PSM collects the scattering light with an air objective placed above the sample, and there is no need for any high NA oil-immersion objective. Owing to the expanded surface plasmons illumination zone (exceeding 1×1cm2), Chip-PSM can realize a large field-of-view to simultaneously detect numerous nanoparticles. Chip-PSM also enables real-time imaging of biological cells or nanoparticles in liquid solution. Additionally, the label-free sensing of hygroscopic growth in single aerosol nanoparticles and chemical reactions in individual perovskite nanocrystals are achieved via the Chip-PSM. In comparison to SPRM and PSM, the Chip-PSM offers a simple, low-cost solution for high-quality and high-throughput imaging of single nanoparticles due to its isotropic point-spread-function and large field-of-view.

Results
Principles of chip-based plasmonic scattering microscopy (Chip-PSM)
As shown in Fig. 1a, Chip-PSM integrates a standard upright microscope with a compact optical illumination device. A real photo of the fabricated compact optical illumination device is shown in Fig. S1a. The optical device consists of an LED source, a polymer layer doped with TiO2 nanoparticles, an all-dielectric 1DPC, and a gold-coated glass slide from the bottom to the top. The LED source (center wavelength at 660nm) with a wavelength bandwidth of ~20nm (Fig. S1b) was used as the light source for illumination. The light from LEDs has low spatial coherence and does not induce speckles on the microscopy images as those induced by the laser beams used in conventional SPRM or PSM9,13,23,24. The polymer layer, doped with dispersed TiO2 nanoparticles (with a diameter of ~60nm) and a thickness of 2μm, is placed above the LED source. The high refractive index and low imaginary characteristic of TiO2 particles increase the scattering efficiency. The well-dispersed TiO2 nanoparticles can uniformly scatter light from the LED source in all directions. An all-dielectric 1DPC is placed on the polymer layer, where its angular-dependent transmission is precisely tailored to function as a momentum-space filter31. The light scattered from the polymer layer can be transmitted through this 1DPC at a designed angular range that is matched with the surface plasmon resonant angle. The thin gold film was coated on a glass substrate that was then placed on the 1DPC. A drop of refractive index matching oil was placed between the 1DPC and the gold film-coated glass substrate. By using this configuration, omnidirectional surface plasmons can be excited at the gold/air interface, which can be used as the near-field illumination field for single nanoparticles, as shown in Fig. 1a and Fig. 1b. There is no need for a high-NA oil-immersion objective or other bulky optical element for the excitation of the surface plasmons, as used in conventional PSM23,24.

To enable the 1DPC to function as a momentum‒space filter, we need to find the best suitable structural parameters of the 1DPC. Using the transfer matrix method32, we determined the suitable parameters for the 1DPC. All dielectric 1DPC is made of an altering layer of SiNx (thickness d=75nm, refractive index n=2.45) and SiO2 (thickness d=110nm, refractive index n=1.46), with 15 pairs in total. The angular-dependent transmittance map of the 1DPC is calculated and shown in Fig. S2. Both the transmittance intensity profile of the 1DPC at 660nm and the angular-dependent reflectance of the gold film-coated glass slide is shown in Fig. 1c. The two calculated curves can be seen as the transmittance and the reflectance spectra in momentum space. The horizontal axis is defined as k/k0=nsin(θ), where n is the refractive index of the glass substrate (~1.5), θ is the incident angle, and k0 is the wavenumber of the light in vacuum. Notably, the momentum space filter made of the 1DPC can be tuned by altering the thickness and refractive index of the dielectric layers, allowing the design of a filter working at various excitation wavelengths.

On the reflectance curve, there is a dip (at k/k0=nsin(θ)=1.04, θ=43.9°) when the incident light is transverse magnetic polarized, which is the typical surface plasmon resonant curve26. The horizontal position of this dip represents the excitation angle of the surface plasmons at the gold/air interface. This dip is clearly within the transmittance peak of the 1DPC. The transverse magnetic polarized light is transmitted through the 1DPC and then excites the surface plasmons to illuminate the single nanoparticles. Since only the transverse magnetic polarized beam can excite surface plasmons at the surface plasmon resonant angle, the transverse electric polarized light is then blocked by using the designed 1DPC (see the right part of Fig. 1c). Furthermore, surface plasmons can be excited only at surface plasmon resonant angles that are larger than the critical angle; thus, the 1DPC was also designed to block or filter the transmitted light at small incident angles, as shown in Fig. 1c. Thus, the momentum-space filtering functions of the designed 1DPC reduce the background and enhance the imaging contrast of the PSM.

Unlike SPRM, which records reflected light, PSM or Chip-PSM images of surface plasmons are scattered by single nanoparticles with an air objective placed on top of the samples. As expected, the flat gold film surface also scatters surface plasmons, creating a background (Eb). Consequently, the Chip-PSM image of single nanoparticles is described by

I= |Eb+Es|2= |Eb|2+2 |Eb||Es| cos(ϕ)+ |Es|2 (1)

where ϕ is the phase difference between light scattered by single nanoparticles and light scattered by a rough surface. Owing to the short distance between the nanoparticles and the rough surface, the phase difference is close to zero, resulting in positive interference contrast, as in PSM or evanescent scattering microscopy23,30. To enhance image contrast, we can subtract the background intensity |Eb|2, which is obtained from areas without nanoparticles. From Eq. (1), when the diameter of the nanoparticles decreases, the interference term becomes dominant. This technique is useful for detecting the tiny signals of single nanoparticles.

For conventional PSMs, single nanoparticles are illuminated by surface plasmons propagating along one direction, and the PSM image of single nanoparticles (diameter at 150nm, n=1.51) is simulated via finite-difference time-domain methods33, as shown in Fig. 1d (the white arrowhead indicates the propagation direction of the excited surface plasmons). The simulated pattern represents the point-spread-function of the PSM, which is noncircular and nonuniform; thus, the image of a single nanoparticle on the PSM image is distorted (the first three images in Fig. 1d). This distortion will be more obvious when the imaged object has complicated shapes, which will be demonstrated in the following experiments on the imaging of curved nanowires.

In contrast, in the case of Chip-PSM, the excited surface plasmons can propagate in all directions along the gold/air interface (omnidirectional surface plasmons), as shown in Fig. 1b; thus, single nanoparticles can be more uniformly illuminated. As a result, the Chip-PSM image of single nanoparticles is circularly symmetric and uniform, as shown in the last image in Fig. 1d, meaning that the Chip-PSM has an isotropic point-spread-function and thus better imaging quality. This simulation result is explained by the discrete dipole approximation (Supplementary Section 1)34.

Isotropic point-spread-function and large field-of-view of the Chip-PSM
To verify the above principle, the 1DPC was fabricated via PECVD (Plasma-Enhanced Chemical Vapor Deposition) of SiO2 and SiNx on a coverslip. The process of PECVD depends on the chemical reaction of SiH4 with N2O and NH3 at high temperature. The refractive index of SiNx can be precisely adjusted by changing the ratio of SiH4 to NH329. The compact optical illumination device consists of a gold-coated glass slide, the 1DPC, a polymer layer, and an LED source. The fabrication details of the compact optical illumination device are provided in Supplementary Section 2. The 1DPC and the polymer layer were characterized by scanning electron microscopy, as shown in Fig. S3. To experimentally demonstrate the functions of the fabricated 1DPC and gold film, we measured the angular-dependent transmittance of the 1DPC and the angular-dependent reflectance of the gold film-coated coverslip (Fig. S4) via a home-built back focal plane imaging setup, as shown in Fig. S5. The back focal plane imaging setup is now widely used for momentum space optical measurements, such as dispersion relation curves, wave vectors of guided modes, and angular distributions of light from various emitters35,36. The experimentally measured transmittance curve of the 1DPC at a wavelength of 660nm is nearly consistent with the theoretical results, as shown in Fig. S6. Then, the fabricated gold film-coated coverslip, the 1DPC, and the nanoparticle-doped polymer film are placed onto a commercial LED source one by one. A drop of index-matching oil (n=1.51) was placed between the 1DPC and the gold film-coated coverslip. The gold film is placed on the top layer to sustain the propagation of surface plasmons at the gold/air interface. The coupling efficiency of the compact optical illumination device is discussed in Supplementary Section 3.

To demonstrate the advantages of the Chip-PSM, we also established the setup of typical PSM, azimuthally-rotation-illumination-PSM, typical SPRM, and azimuthally-rotation-illumination-SPRM, as shown in Supplementary Section 4. In azimuthally-rotation-illumination-PSM and azimuthally-rotation-illumination-SPRM, a pair of galvanometers were used to achieve rotational illumination (Fig. S7). First, single polymer nanoparticles with a diameter of 150nm (n=1.51) were imaged using these configurations as well as Chip-PSM. Figure 2a–c show the experimental results for single polystyrene nanoparticles obtained via conventional PSM, azimuthally-rotation-illumination-PSM, and Chip-PSM, respectively. For all three systems, an air objective (40×, NA=0.6) was used, limiting the resolution to ~550nm (λ/2NA). Therefore, images of single 150nm nanoparticles can be taken as the experimental point-spread-function of the imaging system.

Figure 2a shows the point-spread-function of the conventional PSM with single-direction illumination, which is noncircular or distortional, which is consistent with the finite-difference time-domain simulation results (Fig. 1d). In contrast, the point-spread-function for the azimuthally-rotation-illumination-PSM (Fig. 2b) and Chip-PSM (Fig. 2c) are more symmetric and isotropic. To make a more comprehensive comparison, we also measured the point-spread-function of SPRM and azimuthally-rotation-illumination-SPRM, which exhibited a parabolic tail shape and two closely spaced bright spots, respectively. As shown in Fig. S8a and S8b, the shape of a single nanoparticle in the image is obviously distorted. The comparisons among the images obtained via SPRM (Fig. S8a), azimuthally-rotation-illumination-SPRM (Fig. S8b), PSM (Fig. 2a), azimuthally-rotation-illumination-PSM (Fig. 2b), and Chip-PSM (Fig. 2c) verify that image distortion is much greater for SPRM and azimuthally-rotation-illumination-SPRM and that the Chip-PSM image is distortion-free.

As shown in Fig. S7c, d, for the PSM and azimuthally-rotation-illumination-PSM, the surface plasmons are excited by the high-NA oil-immersion objective lens and work as the illumination field for microscopic imaging. Owing to the limit field-of-view of this oil-immersion objective (100×, NA=1.49), surface plasmons can be excited in a limited area that is smaller than the field-of-view of the imaging objective (air objective, 40×, NA=0.6). Thus, the field-of-view of the PSM and azimuthally-rotation-illumination-PSM is determined by the area of the excited surface plasmons, meaning that the field-of-view cannot be increased even when an air objective is used for imaging. In contrast, surface plasmons can be excited over a large area by using the proposed compact optical illumination device, as demonstrated in Fig. 1a, b; thus, the field-of-view of Chip-PSM can be increased by using an air objective with a low magnification factor. To demonstrate the difference in the field-of-views of the three imaging systems, silver nanowires with a linewidth of 50nm were placed on the gold film and used as the imaging objects. As shown in Fig. 2d, e, the PSM and azimuthally-rotation-illumination-PSM have a field-of-view of ~220μm in diameter; a 100× oil-immersion objective is used to excite the surface plasmons, and a 40× air objective is used to image the nanoparticles (Fig. S7c, d). In contrast, the Chip-PSM has a large illumination zone (larger than 1×1cm2). When the Chip-PSM is equipped with 40× (NA=0.6), 10× (NA=0.3), and 4× (NA=0.2) air objectives, its field-of-view reaches 330×330μm2, 1.32×1.32mm2, and 3.3×3.3mm2, respectively (Fig. 2f–h). Owing to the large field-of-view of the Chip-PSM, many more silver nanowires can be recorded on a single microscopy image, as shown in Fig. 2h. The large field-of-view of the Chip-PSM is favorable for high-throughput optical measurements. It is worth noting that prism-based37 PSM can also achieve a millimeter-scale imaging field of view. However, this approach requires precise control of light coupling into surface plasmons and relies on a bulky prism, which poses challenges for system integration. Additionally, the prism-based system typically excites SPs in a single direction, which can lead to a noncircular or distortional point-spread-function (PSF). In contrast, the proposed Chip-PSM integrates the light source with micro-nanostructures designed for omnidirectional surface plasmons excitation, significantly reducing the system’s footprint and simplifying operational complexity.

In addition to the large field-of-view, the Chip-PSM achieves superior imaging quality compared with the PSM and azimuthally-rotation-illumination-PSM. As indicated by the green arrow-1 in Fig. 2d–f, an alpha-shaped nanowire is fully resolved in the C-PSM image (Fig. 2f), whereas it is partially invisible in the PSM and azimuthally-rotation-illumination-PSM images (Fig. 2d, e). The straight line indicated by the green arrow-2 is nearly invisible in Fig. 2d and can be detected only on the Chip-PSM and azimuthally-rotation-illumination-PSM images (Fig. 2e, f), which directly demonstrates the advantages of azimuthally rotated illuminations and omnidirectional illuminations. The mechanism of this advantage can be briefly explained as follows. When the orientation of the nanowire is along the propagation path of the surface plasmons, its scattering light will be weak, and the light scattered from each point on this nanowire will experience destructive interference; thus, it becomes nearly invisible in the image. When the orientation of the nanowire is perpendicular to the propagation direction of the surface plasmons, the scattering light is stronger, as demonstrated in Fig. 2d; thus, the nanowire is more visible in the image. As a result, when the surface plasmons’ propagation directions are rotating or omnidirectional, the orientation of the nanowire is always perpendicular to the propagation direction of the surface plasmons, which is the reason why the nanowires or various orientations are all visible on the azimuthally-rotation-illumination-PSM and Chip-PSM images (Fig. 2e, f). The advantage of the Chip-PSM over the azimuthally-rotation-illumination-PSM is that there is no need for mechanical scanning or bulky optical setups, resulting in higher temporal resolution, lower cost, and simpler experimental configuration.

In our experiments, we also imaged the nanowires with SPRM and azimuthally-rotation-illumination-SPRM (Fig. S8c, d), where the nanowires were surrounded by considerable background noise and nearly unresolved, indicating that the imaging quality of SPRM is worse than that of PSM. For comparison, we also replaced the gold film-coated coverslip with a bare coverslip for Chip-PSM, and the corresponding images are shown in Fig. S9, which demonstrates that the imaging contrast for single nanoparticles is enhanced if the gold film is used (Supplementary Section 5).

Label-free imaging of single polymer nanoparticles
The advantages of Chip-PSM have been demonstrated, and we verify the detection ability of Chip-PSM by imaging polystyrene nanoparticles with diameters varying from 40–200nm. Polystyrene nanoparticles (n=1.51) were placed on the gold film and illuminated by the surface plasmons at the gold/air interface via the setup shown in Fig. 1a. To obtain a more accurate scattering intensity of polystyrene nanoparticles, we average a sequence of images to suppress random noise and subtract the background (|Eb|2) from the image area without nanoparticles; the image processing is shown in Supplementary Section 6. The Chip-PSM images of polystyrene nanoparticles with diameters of 40, 60, 80, 120, 150, and 200nm are shown in Fig. S10. We determine the scattering intensity of the polystyrene nanoparticles by integrating the pixel intensity within a square area (7×7 pixels; the nanoparticle is located at the center of this area). Image intensity histograms of polystyrene nanoparticles with diameters of 40, 60, 80, 120, 150, and 200nm are constructed (Fig. 3a), which approximately fit with Gaussian distributions12, as indicated by the red lines in Fig. 3a. The deviation of the experimental results from the ideal Gaussian distribution can be attributed to slight variations in nanoparticle sizes, which affect the scattering intensity distribution, as well as the limited number of nanoparticles measured in the experiment.

Curves of the image intensity at the peak of each histogram versus the nanoparticle diameter on a logarithmic scale are plotted in Fig. 3b, and the error bars represent the standard deviations of the Gaussian fits of the corresponding histograms (Fig. 3a). Considering the exponential decay of surface plasmons with increasing distance from the surface, the diameters of these nanoparticles are replaced by the effective diameters (Supplementary Section 7). In the large nanoparticle regime (diameters>90nm), the image intensity follows a power law of d5.8, where the exponent is close to six. This is expected since the light scattered from nanoparticles dominates as the diameter increases, so the measured image intensity is proportional to |Es|2 according to Eq. (1). However, in the small nanoparticle regime, the image intensity is proportional to d3, which is predicted by the interference term 2 |Eb||Es| cos(ϕ) in Eq. (1). The interference effect makes Chip-PSM possible for detecting and imaging smaller nanoparticles. Chip-PSM is also suitable for imaging nanoparticles and biological cells immersed in water (Supplementary Section 8). Two videos show the dynamic motion of the nanoparticles and the biological cells (Movie S1 and Movie S2).

Label-free sensing of the hygroscopic growth of single aerosol nanoparticles
In the following experiments, we verify the sensing ability of Chip-PSM by detecting tiny changes in single aerosol nanoparticles with diameters as small as ~50nm. Atmospheric aerosols can pose significant threats to human health38, as smaller particles such as PM2.5 (particles with a diameter of 2.5 micrometers or less) can penetrate deep into the lungs and even enter the bloodstream. In our previous work, SPRM39 and Bloch surface wave microscopy40 were proposed to detect the hygroscopic growth of single aerosol nanoparticles. However, these methods requires scanning galvanometers, precise optical path alignment, and high-NA oil-immersion objectives to excite surface waves for probing aerosol nanoparticles with diameters of ~90nm. Therefore, developing an imaging system with a simpler configuration for detecting smaller aerosol nanoparticles is necessary and important.

Here, we use Chip-PSM to image and detect the hygroscopic growth of single aerosol nanoparticles with a diameter of ~50nm. The component of aerosol nanoparticles is NaCl, which is the primary chemical component involved in breaking ocean droplets and has a well-established deliquescence relative humidity41. The corresponding optical setup is shown in Fig. S11a, where the aerosol nanoparticles are deposited on the substrate (gold-coated glass slide). A sample chamber was placed on the substrate, as shown in Fig. S11b, and the relative-humidity in the chamber was controlled dynamically from 60 to 87% with an uncertainty of 1% relative-humidity via a custom-made humidification system (Fig. S11c). An air objective is placed on top of the sample chamber to collect the scattered light from the single aerosol nanoparticles, as shown in Fig. 1a. Deposition on the substrate was discussed in our previous work39,40,42, and a differential mobility analyzer was used to screen the desired deposited nanoparticle size (50nm). The scanning electron microscopy image of the deposited NaCl nanoparticles is presented in Fig. 4a, with the size distribution histogram primarily concentrated at 50–60nm (Fig. 4b), which is consistent with the differential mobility analyzer screening size (50nm).

At each relative-humidity value, the residence time is ~15 min to allow sufficient reaction of the aerosol nanoparticles. We averaged a sequence of images and determined the scattering intensity of the aerosol nanoparticles by integrating the pixel intensity within a square area (7×7 pixels). Image processing is like that used for polystyrene nanoparticles. The Chip-PSM image revealed many nanoparticles. Three aerosol nanoparticles with smaller and similar brightness values were selected at a low relative humidity (20%), each with an approximate diameter of 50nm. We averaged the three aerosol nanoparticle intensities as the final value (IRH) at the selected environmental relative-humidity. As shown in Fig. 4c, Chip-PSM images for single aerosol nanoparticles at four relative-humidity setpoints are presented, which reveal that the intensity of the aerosol nanoparticles increases with increasing relative-humidity due to the absorption of water vapor. This demonstrates that the size of aerosol nanoparticles increases during the hygroscopic growth process. The curve of this relative intensity (defined as IRH = IRH/IRH=60%) versus the relative-humidity value is plotted in Fig. 4d. The error bars represent the standard deviation of the three selected nanoparticle intensities. When the relative-humidity increases from 60% to 75%, the relative intensity increases slightly, indicating that the NaCl aerosol nanoparticles do not absorb much water vapor from the air space. However, when the relative-humidity reaches 77%, the scattering intensity increases significantly, which can be explained by the deliquescence of NaCl aerosol nanoparticles. The threshold relative-humidity value is called deliquescence relative humidity, at which time aerosol nanoparticles undergo a solid‒aqueous phase transition. In our experiment, the measured deliquescence relative humidity of pure NaCl aerosol nanoparticles was 77%, which agreed with previous work42,43. To evaluate the accuracy of Chip-PSM in sensing the hygroscopic growth of aerosol nanoparticles, an extended aerosol inorganic model (Fig. 4d, solid red curve) [http://www.aim.env.uea.ac.uk/aim/aim.php] is used to predict hygroscopic growth, which is consistent with the experimental results. The extended aerosol inorganic model prediction results verify that Chip-PSM can image and sense the hygroscopic growth of single NaCl aerosol nanoparticles with diameters of ~50nm.

Enhancing the detection limit of Chip-PSM
Finally, we demonstrate that the detection limit of the proposed Chip-PSM can be further enhanced through optimization of the chip structures. We added a layer of silicon (thickness: 8nm) on top of the gold film. The red lines in Fig. 5a show the calculated reflectivity spectra of conventional surface plasmon resonant sensors (the gold film) and silicon-enhanced surface plasmon resonant sensors as a function of k/k0=nsin(θ). The solid red line indicates that the conventional surface plasmon resonant sensor has a full width at half maximum of ~0.03 (Δθ=1.6°). Moreover, the dashed red line shows that the reflectivity spectrum of the silicon-enhanced surface plasmon resonant sensors has a broader full width at half maximum of ~0.15 (Δθ=9.4°). The calculated transmittance spectrum of the designed 1DPC is also plotted in Fig. 5a with a dark line. These results indicate that, compared with conventional surface plasmon resonant sensors, surface plasmons in silicon-enhanced surface plasmon resonant sensors can be excited over a wider incident angle, improving the energy efficiency of the light transmitted from the 1DPC and the detection limit for single nanoparticles. The calculation results can be attributed to the change in the effective refractive index on the air side after the silicon film is added to the gold film44,45. The experimental reflectivity spectra of the silicon-enhanced surface plasmon resonant sensors are shown in Fig. S4f and are nearly consistent with the calculated results.

Owing to the broad angular range of surface plasmons’ excitation, enhanced Chip-PSM enables the detection and imaging of smaller nanoparticle signals over a large field-of-view. Moreover, the silicon-enhanced surface plasmon resonant sensors have a higher excitation angle (at k=k0=1.19, θ=52.5°) than conventional surface plasmon resonant sensors do, allowing PSM imaging with a higher NA air objective. As shown in Fig. 5b, single gold nanoparticles with a diameter of ~10nm are imaged via Chip-PSM with a 40× (NA=0.6) air objective, achieving a field-of-view of 275×275μm2. The very bright spots in Fig. 5b are signals from large particles originally present on the substrate. The enlarged images of the 10nm gold nanoparticles (marked with green boxes) are shown in Fig. 5c and can be clearly detected and individually imaged. Fig. 5b, c demonstrate that Chip-PSM can detect and image many nanoparticles in real time without image post-processing, which is challenging for traditional PSM. Increasing the exposure time allows the detection and imaging of weaker signals, such as the scattering light from single 30nm polystyrene nanoparticles and 5nm gold nanoparticles, as shown in Fig. S12.

Finally, the Chip-PSM was used to detect and image anion exchange reactions in single cesium halide perovskite nanocrystals. These materials are known for their outstanding photoelectric conversion properties, making them highly relevant for applications in solar cells and photodetectors46–48. Anion exchange in cesium lead halide perovskites enables tuning of photoluminescence across the visible spectrum while preserving high quantum yields and narrow emission linewidths49. In the experiment, cesium lead halide perovskite (CsPbX3, X=Cl, I, Br) nanoparticles are spin-coated on the substrate (silicon-coated gold film). The transmission electron microscopy image, shown in Fig. S13e, revealed an edge length of ~10nm. Fig. 5d shows a PSM image of CsPbI3 nanoparticles without image postprocessing. After HCl vapor is introduced to the substrate that is spin-coated with CsPbI3 nanoparticles, the intensity of the single CsPbX3 nanoparticles significantly decreases due to the anion exchange reaction, which converts the CsPbI3 nanoparticles to CsPbCl3, as shown in Fig. 5e. In the next step, HI vapor is added, and another anion-exchange reaction occurs, in which the nanoparticles are recovered from CsPbCl3 to CsPbI3. The intensity of the single nanoparticles increases again, as shown in Fig. 5f. The experimental results are consistent with our previous work50, demonstrating that Chip-PSMs can detect and image the chemical reactions of single nanoparticles with high sensitivity. Notably, the perovskite nanoparticles on the substrate were exposed to HCl or HI for ~30 min prior to Chip-PSM imaging under the assumption that the ion exchange reaction had fully occurred. Direct introduction of acidic gas onto the perovskite surface will damage the air objective used in Chip-PSM. However, the use of an acid-resistant sample chamber in combination with Chip-PSM could enable real-time observation of the ion exchange process.

In fact, the existing imaging system can be entirely replaced by compact imaging devices such as smartphones, microlenses, or metalenses, enabling the full integration and portability of the PSM system. To verify the compatibility of compact imaging devices with the compact optical illumination device, we have demonstrated the use of a smartphone and a small lens as the imaging system for the Chip-PSM and successfully imaged silver nanowires with a diameter of 70nm. The results are showed in supplementary section 9.

Methods
Optical experimental setup
All Chip-PSM images were taken on a modified optical microscope (Nikon Ni-U) using an air objective (CFI Super Plan Fluor ELWD and 40×, NA=0.6, WD=3.6–2.8mm), except for Fig. 2g (CFI Plan Fluor DL and 10×, NA=0.3, WD=15.2mm) and Fig. 2h (CFI Plan Apochromat Lambda D 4×, NA=0.2, WD=20.0mm), for which additional air objectives were used. All Chip-PSM images were recorded by Prime 95B sCMOS (TELEDYNE PHOTOMETRICS). The LED light source was purchased from Shanghai Xilong Optoelectronics Technology Co., Ltd., with a model specification of LED09-660-1W. The total optical power is ~1W, and the optical power density incident on the polymer layer is ~1W cm−2.

All the BFP measurements were performed on a modified optical microscope (Nikon Ti2-U). An oil-immersion objective (CFI Apochromat TIRF 100×, NA=1.49, WD=0.12mm) from Nikon Japan was used to measure the angular distribution on the BFP fully. A broadband white light source with a bandpass filter was used to measure the reflectivity BFP (Fig. S5b). Three filters were used to measure the transmission BFP of the 1DPC, where the source was the LED source used in the experiment. The Neos CMOS detector for recording the BFP images was from Andor Oxford Instruments (UK).

Preparation of the nanoparticles and nanowires
The polystyrene nanoparticles were purchased from Thermo Fisher Scientific (USA). The certified mean diameters of the nanoparticles supplied were ~30, 40, 60, 80, 120, 150, and 200nm. Gold nanoparticles with diameters of 5nm and 10nm, as well as silver nanowires with a diameter of 50nm and 70nm, were purchased from XFNANO Materials Tech. Co., Ltd. These gold nanoparticles, polystyrene nanoparticles and sliver nanowires were diluted in anhydrous ethanol and dropped on the substrate to dry. To image gold nanoparticles in water, the nanoparticles are first dissolved in deionized water, after which a drop of the solution is placed on the surface of the gold film and covered with a clean coverslip. Transmission electron microscopy images of Au nanoparticles with diameters of 5nm, 10nm, 20nm, and 50nm, along with scanning electron microscopy images of polystyrene nanoparticles with sizes of 30nm, 40nm, 60nm, 80nm, 120nm, 150nm, and 200nm, as well as silver nanowires (m) with a linewidth of 50nm, are presented in Fig. S13. The fabrication details of the CsPbI3 nanoparticles were presented in our previous work50.

Preparation of neutrophil samples
A commercial neutrophil extraction kit was used to isolate neutrophils from 1mL of whole blood, followed by dilution to 1×106 cells/mL in culture medium containing 5% fetal bovine serum. The neutrophils were then incubated at 37°C in a 5% CO2 incubator for 30 min. Then, the cell suspension was quickly dropped onto the Au film surface, and another piece of glass slide was used as the coverslip, which was placed on the suspension and flattened.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "General comments\nThe paper deals with the improvement of the field of view in plasmonic-assisted single molecule detection. Specifically,\nauthors have developed a new chip enabling an isotropic point-spread-function for enhanced image quality.\nThe overall paper is very well written and clear and as far as I have seen exempt of any mistake.\nThe article presents accurate design validated by careful experiments demonstrating the concept. This study represents an\nimportant step forward in the field of single molecule detection and therefore I recommend the paper to be accepted after\nconsidering the few minor following comments.\nComments and suggestions:\n1) Lines 111-112: “… a well designed…”: this is a bit vague. I would suggest to reformulate.\n2) Line 124: The refractive index of silicon nitride at 660 nm seems to be very high. 2.45 would correspond to titania not\nsilicon nitride. I wonder if the authors have inverted the values. It should be closer to 2.0x.\n3) Fig 1d is cited before Fig 1c.\n4) Line 173: The standard deposition method can be stated in the main text.\n5) Line 181-183: Fig 1b is only the simulation, while the text talk about experimental results nearly matching the simulation. I\nsuggest showing this transmission curve too (Fig 5a?).\n6) Fig 2: scale bar could be added to a, b, d, and e for clarity.\n7) I am curious about the validity of the Gaussian fits in Fig. 3. They do not look as good as it should.\n8) Finally, I recommend to carefully check the editing of the equations.", "reviewer_2": "The manuscript reports a compact chip-based omnidirectional plasmonic illumination system that enables plasmonic\nscattering microscopy (PSM) of nanoparticles with an isotropic point spread function and a large field of view. The authors\nalso demonstrated the capability of this system for detecting 30 nm dielectric nanoparticles, 5 nm AuNPs, and chemical\nreactions of single nanocrystals. The innovative compact evanescent illumination design consists of a red LED source, a\nTiO2 nanoparticle doped polymer layer to create a uniform scatter light, an all-dielectric multilayer photonic crystal (1DPC)\nas a momentum-space filter to transmit the light around the surface plasmon resonance angle, and a think gold film coated\nglass substrate to generate the plasmonic field. An optional SiO2 coating on top of the gold film can broaden the plasmonic\nresonance peak to allow more light coupled to the sample space and thus enhance the illumination intensity. This design\nsignificantly reduces the size (and maybe cost as well) of the plasmonic illumination hardware and provides a large field-of-\nview illumination with no fringe pattern, which could lead to a portable system for field applications. The major limitation of\nthis design is that the LED provides limited power density that prevents this device from imaging single molecules.\nConsidering the major application of PSM is for single molecule detection, comparing this device with PSM seems a bit\nmisleading. In addition, the imaging portion of the setup is still the same as the standard PSM with an objective, a tube lens,\nand a digital camera, which is still bulky. The demonstrated applications do not truly showcase the advantage of the chip\ndesign, as they all could be done on a prism-couped PSM. Nevertheless, I think the manuscript may be worth publishing for\nthe innovation mentioned above after addressing the following questions/comments:\n1. Could a portable imaging system, such as a smartphone with a microlens be used with the chip? This will make a truly\nportable device and could lead to some unique applications.\n2. Some experimental details are not provided. What is the source and model number, total power and power density of the\nLED? What is the coupling efficiency of the chip?\n3. In lines 212-215, the authors state that the field of view of objective-based PSM is limited. This should be clarified.\nBecause SPR and PSM can easily reach a field-of-view over 1mm2 with a prism-coupled configuration, e.g. Angew. Chem.\nInt. Ed. 2022, e202209469.\n4. In supplementary section 5, more detailed information about image processing should be provided for direct comparison.\nThis includes the number of averaged frames, the size of the median filter, and the value of the threshold. Before drift\ndominates the noise, increased averaging will undoubtedly improve signal-to-noise ratio.\n5. Line 348 mentioned Fig. S5f, which cannot be found in the supporting info.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "We thank all reviewers for their valuable comments on our manuscript. A point-by-point response\nto these comments with manuscript modifications is provided below.\nREVIEWER COMMENTS\nReviewer #1:\nGeneral comments:\nThe paper deals with the improvement of the field of view in plasmonic-assisted single molecule\ndetection. Specifically, authors have developed a new chip enabling an isotropic point-spread-\nfunction for enhanced image quality.\nThe overall paper is very well written and clear and as far as I have seen exempt of any mistake.\nThe article presents accurate design validated by careful experiments demonstrating the concept.\nThis study represents an important step forward in the field of single molecule detection and\ntherefore I recommend the paper to be accepted after considering the few minor following comments.\nResponse: Thank you for your concise summary of our work and your positive comments on our\nmanuscript. We have responded the comments point by point as follows.\nComments and suggestions:\n1) Lines 111-112: “… a well designed…”: this is a bit vague. I would suggest to reformulate.\nResponse: We sincerely thank the reviewer for pointing out the vagueness in the phrase \"a well-\ndesigned all-dielectric 1DPC\". As suggested, we have reformulated this expression to provide a more\nprecise and specific description in the revised manuscript.\nFor your convenience, we copy these contents here.\nOn page 6 \"The well-dispersed TiO nanoparticles can uniformly scatter light from the LED source in\n2\nall directions. An all-dielectric 1DPC is placed on the polymer layer, where its angular-dependent\ntransmission is precisely tailored to function as a momentum-space filter32. The light scattered from\nthe polymer layer can be transmitted through this 1DPC at a designed angular range that is matched\nwith the surface plasmon resonant angle. \"\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\n2) Line 124: The refractive index of silicon nitride at 660 nm seems to be very high. 2.45 would\ncorrespond to titania not silicon nitride. I wonder if the authors have inverted the values. It should be\ncloser to 2.0x.\nResponse: Thank you very much for your valuable suggestion and questions, we are sorry that we\nhave no clearly described why the refractive index of silicon nitride can be 2.45. To clarify, we have\nrevised the material designation from \"Si N \" to \"SiN \", as the refractive index of SiN can vary\n3 4 X X\ndepending on the deposition conditions. Indeed, the process of PECVD (Plasma-Enhanced Chemical\nVapor Deposition) involves the chemical reaction of SiH with N O and NH at high temperatures. By\n4 2 3\nadjusting the ratio of SiH to NH , the refractive index of SiN can be precisely tuned over a wide range.\n4 3 X\nIn our work, a higher proportion of SiH was used to achieve a high refractive index for the SiNx layer.\nFor your convenience, we have copied it as follows:\nOn page 8 \" Isotropic point-spread-function and large field-of-view of the Chip-PSM. To verify the\nabove principle, the 1DPC was fabricated via PECVD (Plasma-Enhanced Chemical Vapor Deposition)\nof SiO and SiN on a coverslip. The process of PECVD depends on the chemical reaction of SiH with\nN O and NH at high temperature. The refractive index of SiN can be precisely adjusted by changing\n2 3 X\nthe ratio of SiH to NH 29. The compact optical illumination device consists of a gold-coated glass slide,\n4 3\nthe 1DPC, a polymer layer, and an LED source. The fabrication details of the compact optical\nillumination device are provided inSupplementary Section 2. The 1DPC and the polymer layer were\ncharacterized by scanning electron microscopy, as shown in Fig. S3. \"\n29. Kuai Y, Chen J, Fan Z, Zou G, Lakowicz JR, Zhang D. Planar photonic chips with tailored angular\ntransmission for high-contrast-imaging devices. Nature Communications12, 6835 (2021).\n3) Fig 1d is cited before Fig 1c.\nResponse: Thank you for your kind reminder. In the original manuscript,Fig. 1cwas first cited on line\n118, which preceded the citations ofFig. 1bandFig. 1d. To ensure the correct order of figure citations,\nwe have swapped the labels ofFig. 1bandFig. 1cin the revised manuscript. Additionally, we have\nchecked the order of all figures to ensure their correct referencing.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\n4) Line 173: The standard deposition method can be stated in the main text.\nResponse: Thank you for your kind reminder. We appreciate your suggestion to clarify the deposition\nmethod in the main text. We sincerely apologize for the inaccurate description in the original\nmanuscript. In fact, the1DPC(one dimensional photonic crystal) was fabricated using a standard\ndeposition PECVD (Plasma-Enhanced Chemical Vapor Deposition) process, not the proposed compact\noptical illumination device. We have changed this inaccurate description in the revised manuscript.\nAs for the description of the PECVD process, we have already provided detailed information in our\nresponse to the second question. Thank you again for your constructive comment.\n5) Line 181-183: Fig 1b is only the simulation, while the text talk about experimental results nearly\nmatching the simulation. I suggest showing this transmission curve too (Fig 5a?).\nResponse: Thank you very much for your valuable suggestion and questions. We sincerely apologize\nfor the deficiencies in our original description. The experimental transmittance map of the 1DPC and\nthe reflectance map of the gold film are shown in Fig. S4. The transmission curve of the 1DPC can be\nextracted by drawing a cross-section line along the polarizer direction on the experimental\ntransmittance map. We have added the experimentally measured and theoretically calculated\ntransmittance curves of the 1DPC, as shown in Fig. S5.For your convenience, we have copied these\ncontents here.\nOn page 9 \" The back focal plane imaging setup is now widely used for momentum space optical\nmeasurements, such as dispersion relation curves, wavevectors of guided modes, and angular\ndistributions of light from various emitters 1, 2. The experimentally measured transmittance curve of\nthe 1DPC at a wavelength of 660 nm is nearly consistent with the calculated results, as shown in Fig.\nS6. Then, the fabricated gold film-coated coverslip, the 1DPC, and the nanoparticle-doped polymer\nfilm are placed onto a commercial LED source one by one. \"\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nFig. S6. The experimentally measured and theoretically calculated transmittance curves of the 1DPC\nat 660 nm.\n6) Fig 2: scale bar could be added to a, b, d, and e for clarity.\nResponse: Thank you for your kind reminder. We have added scale bars toFig. 2a, 2b, 2d, and 2ein\nthe revised manuscript.\nFor your convenience, we copy these figures here.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nFig. 2 The point-spread-function and field-of-view of three types of PSMs. a-c, Experimental images\nof single polystyrene nanoparticles (diameter of 150 nm, n = 1.51) obtained via PSM, Azimuthally-\nRotation-Illumination-PSM, and Chip-PSM. Scale bar, 3.3 μm. d-f, Experimental images of silver\nnanowires obtained via PSM, Azimuthally-Rotation-Illumination-PSM, and Chip-PSM. Scale bar, 33 μm.\nAn air objective (40×, NA=0.6) is used for capturing images on (a-f). The white arrowhead in Fig. 2a\nand 2d represents the propagation direction of the excited surface plasmons. g-h, Chip-PSM images\nof silver nanowires. Air objectives of 10× (NA=0.3) and 4× (NA=0.2) were used for image on (g), and\n(h), respectively. The zone marked with a white box on (g-h) represents the same imaging areas as\nshown in Fig. 2d-f. The scale bars represent 132 μm in Fig. 2g and 330 μm in Fig. 2h.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\n7) I am curious about the validity of the Gaussian fits in Fig. 3. They do not look as good as it should.\nResponse: Thank you for your insightful comment. We agree that the Gaussian fits do not appear\nideal, and this can be attributed to several factors:\n1. Nanoparticle size variations: The actual sizes of the nanoparticles have slight deviations, which\ncan affect the scattering intensity distribution.\n2. Limited number of nanoparticles: The scattering intensities of nanoparticles with different\ndiameters were measured from three captured images, and the limited number of nanoparticles\nmay have resulted in a less-than-ideal Gaussian fit.\n3. Non-uniform gold film surface: The rough surface of the gold film causes variations in the z-axis\ndistances of the nanoparticles, leading to fluctuations in the evanescent field illumination\nintensity. This will affect the scattering intensity distribution of nanoparticles.\nWe think that these factors contribute to the deviation from a perfect Gaussian distribution.\nNevertheless, the Gaussian fit provides a reasonable approximation for the overall trend in the\nscattering intensity distribution. We have added some descriptions on this point in the revised\nmanuscript. For your convenience, we copy them as follows:\nOn page 13 \" Image intensity histograms of polystyrene nanoparticles with diameters of 40, 60, 80,\n120, 150, and 200 nm are constructed (Fig. 3a), which approximately fit with Gaussian distributions12,\nas indicated by the red lines in Fig. 3a. The deviation of the experimental results from the ideal\nGaussian distribution can be attributed to slight variations in nanoparticle sizes, which affect the\nscattering intensity distribution, as well as the limited number of nanoparticles measured in the\nexperiment.\"\n12. Wang S, et al. Label-free imaging, detection, and mass measurement of single viruses by surface\nplasmon resonance. Proceedings of the National Academy of Sciences107, 16028-16032 (2010).\n8) Finally, I recommend to carefully check the editing of the equations.\nResponse: Thank you for your kind reminder. We have carefully checked the editing of all equations\nin the revised manuscript and the supplementary materials.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nReviewer #2:\nGeneral comments:\nThe manuscript reports a compact chip-based omnidirectional plasmonic illumination system that\nenables plasmonic scattering microscopy (PSM) of nanoparticles with an isotropic point spread\nfunction and a large field of view. The authors also demonstrated the capability of this system for\ndetecting 30 nm dielectric nanoparticles, 5 nm AuNPs, and chemical reactions of single nanocrystals.\nThe innovative compact evanescent illumination design consists of a red LED source, a TiO2\nnanoparticle doped polymer layer to create a uniform scatter light, an all-dielectric multilayer\nphotonic crystal (1DPC) as a momentum-space filter to transmit the light around the surface plasmon\nresonance angle, and a think gold film coated glass substrate to generate the plasmonic field. An\noptional SiO2 coating on top of the gold film can broaden the plasmonic resonance peak to allow\nmore light coupled to the sample space and thus enhance the illumination intensity. This design\nsignificantly reduces the size (and maybe cost as well) of the plasmonic illumination hardware and\nprovides a large field-of-view illumination with no fringe pattern, which could lead to a portable\nsystem for field applications. The major limitation of this design is that the LED provides limited power\ndensity that prevents this device from imaging single molecules. Considering the major application of\nPSM is for single molecule detection, comparing this device with PSM seems a bit misleading. In\naddition, the imaging portion of the setup is still the same as the standard PSM with an objective, a\ntube lens, and a digital camera, which is still bulky. The demonstrated applications do not truly\nshowcase the advantage of the chip design, as they all could be done on a prism-couped PSM.\nNevertheless, I think the manuscript may be worth publishing for the innovation mentioned above\nafter addressing the following questions/comments:\nResponse: Thank you for your thorough and insightful review of our manuscript. We appreciate your\nrecognition of the innovative aspects of our compact Chip-PSM and its potential for enabling large\nfield-of-view imaging with an isotropic point spread function. PSM is a powerful technique that\nretains the advantages of SPRM while improving its point spread function (PSF). It also offers higher\ncontrast and resolution for detecting small nanoparticles. Although Pengfei Zhang et al. (Nature\nMethods17, 1010-1017 2020; Angew. Chem. Int. Ed. 2022, e202209469) primarily demonstrated PSM\nin the context of single-molecule detection, we believe that PSM can be applied to other single-\nnanoparticle detection fields, such as single aerosol nanoparticle analysis and the monitoring of\nsingle-nanoparticle formation in chemical reactions. In fact, our approach further optimizes the\nexcitation mechanism of surface plasmon polaritons (SPPs) and refines the imaging PSF in PSM.As\nthe reviewer pointed out, we have innovatively utilized a red LED source, a TiO doped polymer layer,\n2\nand a one-dimensional photonic crystal to excite plasmonic waves on the gold film surface, forming\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\na compact optical illumination device. This excitation method eliminates the need for bulky oil-\nimmersion objectives or prism-based setups, as well as complex optical alignment, significantly\nreducing the system’s size and operational complexity. Moreover, due to the polymer layer in our\ndesign, surface plasmons can be excited in all directions. Consequently, unlike objective- or prism-\nbased PSM systems, the point spread function (PSF) of the Chip-PSM is symmetric, resulting in\nimproved imaging quality. We acknowledge that the prism-based PSM can also achieve large field of\nview. However, it is worth emphasizing that the proposed Chip-PSM offers advantages in integration,\nease of operation, and user-friendliness for non-experts in optics.To further highlight the integration\nadvantages of the proposed Chip-PSM, we replaced the imaging system within the Chip-PSM with a\nsmartphone and a small lens. This experiment demonstrated that the Chip-PSM can achieve portable\nand highly integrated imaging. In the future, the imaging system’s objective, tube lens, and digital\ncamera can be replaced with a microlens array and a CMOS detector, enabling full integration of the\nChip-PSM system.\n1. Could a portable imaging system, such as a smartphone with a microlens be used with the chip?\nThis will make a truly portable device and could lead to some unique applications.\nResponse: Thank you very much for your valuable comments. Yes, this is entirely possible. In the\nmanuscript, we primarily demonstrate the integration of PSM in the illumination module. In fact, the\nexisting imaging system can be entirely replaced by compact imaging devices such as smartphones,\nmicrolenses, or metalenses, enabling the full integration and portability of the PSM system. For\nexample, the compact optical illumination device can be combined with the microlens array imaging\ntechnique described in the literature (Optica 11, 860-871 ,2024) to achieve integrated and portable\nwide-field imaging, which is one of our future research directions. To verify the compatibility of\ncompact imaging devices with the compact optical illumination device, we have added experiments\nin the revised manuscript and appendix showcasing the Chip-PSM system utilizing a smartphone and\na single lens for imaging silver nanowires with a diameter of 70 nm.\nFor your convenience, we copy these contents here.\nOn page 19: \"However, the use of an acid-resistant sample chamber in combination with Chip-PSM\ncould enable real-time observation of the ion exchange process.\nIn fact, the existing imaging system can be entirely replaced by compact imaging devices such as\nsmartphones, microlenses, or metalenses, enabling the full integration and portability of the PSM\nsystem. To verify the compatibility of compact imaging devices with the compact optical illumination\ndevice, we have demonstrated the use of a smartphone and a single lens as the imaging system for\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nthe Chip-PSM and successfully imaged silver nanowires with a diameter of 70 nm. The results are\nshowed in supplementary section 9.”\nSupplementary Section 9: Integration of the compact optical illumination device with a smartphone\nand a small lens.\nFig. S19. (a)Photographs of the smartphone, small lens, and the compact optical illumination device,\nwhich consists of a gold film, 1DPC, polymer layer, and LED source. (b) Experimental setup of the\nsmartphone equipped with a small lens using the compact optical illumination device. (c), (d) Imaging\nresults of silver nanowires obtained using the compact illumination device with both a smartphone\nequipped with a small lens (c) and a standard upright microscope (d). The exposure times were 25 ms\n(c) and 1 ms (d), respectively. The scale bars in (c) and (d) represent 175 µm and 50 µm, respectively.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nTo further miniaturize and enhance the portability of Chip-PSM, a smartphone equipped with a small\nlens was used to replace the standard upright microscope. Fig. S19a shows real photos of the\nsmartphone, small lens, and the compact optical illumination device, which consists of a gold film,\n1DPC, polymer layer, and LED source. The smartphone model used is the Realme GT6, and the small\nlens was purchased from Taobao, China (cost: approximately $26). Fig. S19billustrates the imaging\noptical path: light from the LED source passes through the compact illumination device, exciting\nsurface plasmons (SPs) to illuminate 70 nm diameter silver nanowires placed on the gold film surface.\nThe small lens is attached to the smartphone camera, which captures the scattered light from the\nsilver nanowires. Fig. S19cshows the imaging results obtained with the smartphone,\nwhileFig.S19dpresents the corresponding area results obtained using a standard upright microscope\nfor comparison, equipped with a 10× objective lens (NA = 0.3) and a Prime 95B sCMOS camera\n(TELEDYNE PHOTOMETRICS). This setup demonstrates that integrating a smartphone with a small lens\ncan truly achieve miniaturization and portability for Chip-PSM.\n2. Some experimental details are not provided. What is the source and model number, total power\nand power density of the LED? What is the coupling efficiency of the chip?\nResponse: Thank you very much for your kind reminder. We sincerely apologize for the omission of\nthese experimental details in the original manuscript. We have provided the experimental details as\nfollows:\n1. LED Source: The LED light source was purchased fromShanghai Xilong Optoelectronics\nTechnology Co., Ltd., with a model specification ofLED09-660-1W. The total optical power is\napproximately1 W, and the optical power density incident on the polymer layer is about1 W/cm².\n2. Coupling Efficiency of the Chip: Because surface plasmons (SPs) are localized at the gold film\nsurface, it is not feasible to directly measure the coupling efficiency of the compact optical\nillumination device for SPs excitation. To estimate it, we measured the transmitted light power\nthrough the 1DPC with the polymer layer and compared it to a bare glass substrate. The results\nshowed a transmission of 0.110 mW for the 1DPC and 0.293 mW for the bare glass, corresponding\nto a coupling efficiency of approximately 37.5%. To further enhance the coupling efficiency for SPs\nexcitation, a silicon layer can be deposited on the gold film surface, as discussed in the main text.\nWe have added these details to the revised manuscript. For your convenience, we copy these\ncontents here.\nOn page 9 \"A drop of index-matching oil (n=1.51) was placed between the 1DPC and the gold film-\ncoated coverslip. The gold film is placed on the top layer to sustain the propagation of surface\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nplasmons at the gold/air interface. The coupling efficiency of the compact optical illumination device\nis discussed in Supplementary Section 3. \"\nOn page 20 \"Optical experimental setup. All Chip-PSM images were taken on a modified optical\nmicroscope (Nikon Ni-U) using an air objective (CFI Super Plan Fluor ELWD and 40×, NA = 0.6, WD =\n3.6–2.8 mm), except for Fig. 2g (CFI Plan Fluor DL and 10×, NA = 0.3, WD = 15.2 mm) and Fig. 2h (CFI\nPlan Apochromat Lambda D 4×, NA = 0.2, WD = 20.0 mm), for which additional air objectives were\nused. All Chip-PSM images were recorded by Prime 95B sCMOS (TELEDYNE PHOTOMETRICS).The LED\nlight source was purchased fromShanghai Xilong Optoelectronics Technology Co., Ltd., with a model\nspecification ofLED09-660-1W. The total optical power is approximately1 W, and the optical power\ndensity incident on the polymer layer is approximately1 W cm-2. \"\nSupplementary Section 3: Coupling efficiency measurement.\nFig. S16. Experimental setup for measuring the coupling efficiency of the compact illumination\ndevice. Since surface plasmons (SPs) are confined to the gold film surface, directly measuring the\ncoupling efficiency of the compact optical illumination device for SPs excitation is impractical. To\naddress this, we removed the gold film and measured the transmitted light energy through the\nremaining structure (1DPC and polymer layer), comparing it to transmission through a bare glass\nsubstrate. The experimental setup is shown inFig. S16. The light from the LED source was incident\nonto two substrates, one is the 1DPC with the polymer layer, and the other is bare glass. The LED\noutput power was maintained at a constant level throughout the measurements. Optical power\nmeasurements revealed transmission values of 0.110 mW for the 1DPC with the polymer layer and\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\n0.293 mW for the bare glass, corresponding to a coupling efficiency of approximately 37.5%. A portion\nof the emitted light from the 1DPC contributes to SPs excitation. To further enhance the coupling\nefficiency for SPs excitation, a silicon layer can be deposited on the gold film surface, as detailed in\nthe \"Enhancing the Detection Limit of Chip-PSM\" section of the main text.\n3. In lines 212-215, the authors state that the field of view of objective-based PSM is limited. This\nshould be clarified. Because SPR and PSM can easily reach a field-of-view over 1mm2 with a prism-\ncoupled configuration, e.g. Angew. Chem. Int. Ed. 2022, e202209469.\nResponse: Thank you very much for your valuable suggestion. We have carefully reviewed the\nreference provided by the reviewer. In the work of P. Zhang et al., a prism-coupled SPR system was\nused to construct a PSM system, which indeed achieves a larger field-of-view (over 1 mm²) compared\nto the oil-immersion objective-coupled configuration. This approach is particularly useful for\napplications such as in situ membrane protein binding kinetic analysis over a large field of view.\nHowever, there are several critical distinctions between their work and our proposed Chip-PSM\nsystem, as highlighted below:\n1. Size and Complexity: The prism-coupled configuration is relatively bulky and cumbersome,\nrequiring precise optical alignment to excite surface plasmons (SPs). In contrast, our Chip-PSM\nsystem utilizes a compact optical illumination device, which significantly reduces the size and\ncomplexity of the setup. This makes our system more user-friendly and easier to operate,\nespecially for untrained users.\n2. Imaging Quality: The prism-coupled system typically excites SPs in a single direction, which can\nlead to a noncircular or distortional point-spread-function (PSF). In contrast, our Chip-PSM system\nemploys omnidirectional SPs illumination, which provides an isotropic PSF. This significantly\nimproves imaging quality.\nIn summary, while the prism-coupled PSM system offers a larger field-of-view compared to traditional\noil-immersion objective-coupled systems, our Chip-PSM system provides a more compact, user-\nfriendly, and high-quality imaging solution with an isotropic PSF. These advantages make Chip-PSM a\npromising tool for high-throughput, label-free imaging and sensing of nanoparticles in physical,\nenvironmental, and chemical sciences.\nWe have added the difference between prism-based PSM and Chip-PSM in the revised manuscript.\nFor your convenience, we copy this section as follows:\nOn page 11: \" Owing to the large field-of-view of the Chip-PSM, many more silver nanowires can be\nrecorded on a single microscopy image, as shown in Fig. 2h. The large field-of-view of the Chip-PSM\nis favorable for high-throughput optical measurements. It is worth noting that prism-based PSM38 can\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nalso achieve a millimeter-scale imaging field of view. However, this approach requires precise control\nof light coupling into surface plasmons and relies on a bulky prism, which poses challenges for system\nintegration. Additionally, the prism-based system typically excites SPs in a single direction, which can\nlead to a noncircular or distortional point-spread-function (PSF). ln contrast, the proposed Chip-PSM\nintegrates the light source with micro-nano structures designed for omnidirectional surface plasmons\nexcitation, significantly reducing the system's footprint and simplifying operational complexity.\"\n38. Zhang P, et al. In Situ Analysis of Membrane-Protein Binding Kinetics and Cell–Surface Adhesion\nUsing Plasmonic Scattering Microscopy. Angewandte Chemie International Edition 61, e202209469\n(2022).\n4. In supplementary section 5, more detailed information about image processing should be provided\nfor direct comparison. This includes the number of averaged frames, the size of the median filter, and\nthe value of the threshold. Before drift dominates the noise, increased averaging will undoubtedly\nimprove signal-to-noise ratio.\nResponse: Thank you very much for your kind reminder. We sincerely apologize for not providing a\nclear description of the image processing steps. In the revised manuscript, we have included detailed\ninformation on the number of averaged frames, median filter size, and threshold values.\nWe agree that increasing the number of averaged frames improves the signal-to-noise ratio before\ndrift dominates the noise. However, it is important to note that image processing is primarily used to\nfacilitate the calculation of scattering intensities from a large number of nanoparticles and to generate\nstatistical histograms, as presented in Section 3 of the main text. Other Chip-PSM imaging results,\nsuch as those in Fig. 2, Fig. 4c, and Fig. 5, were obtained without any image processing or averaging.\nAdditionally, compared to traditional PSM, which employs high-power light sources, Chip-PSM utilizes\na lower-power LED source, requiring a longer exposure time to achieve a sufficient signal-to-noise\nratio.\nFor your convenience, we have copied the revised supplementary section here.\nSupplementary Section 6: Image processing for extracting nanoparticle intensity\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nFig. S17 Image processing protocol for obtaining the intensity histograms of polystyrene\nnanoparticles. Scale bars = 2.75 μm.\nAs shown in Fig. S17, image processing involves the following steps:\n1) The raw image sequence is captured via Chip-PSM. The image mechanical drift is eliminated with\nthe image stabilizer plugin in ImageJ. To suppress random noise, the drift-corrected image\nsequence is averaged over ten frames to obtain a mean image. The background ( ) is\nsubtracted from the mean image, which is obtained from the mean image area wit2hout\n|Eb|\nnanoparticles.\n2) A 3×3 median filter in ImageJ is applied to minimize digital noise.\n3) The Chip-PSM intensity of the nanoparticles is calculated by integrating the pixel intensity within\na square area (7×7 pixels). To achieve this, the filtered mean image is convolved with a 7×7 kernel,\nwhere all elements are set to one (stride = 1), to obtain an integrated image.\n4) In the integrated image, the nanoparticles are clearly distinguishable. We manually set the lower\nthreshold as the minimum scattering intensity of the nanoparticles and the upper threshold as\nthe maximum scattering intensity of the nanoparticles to ensure accurate identification. By\nemploying an algorithm to find local maxima 3, we identify the local maximum pixel in the\nintegrated image, where the value represents the intensity of the nanoparticles.\n5) The local maximum value is counted, and the distribution histogram for the nanoparticles is\nplotted.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\n5. Line 348 mentioned Fig. S5f, which cannot be found in the supporting info.\nResponse: We sincerely apologize for this error in referencing. The correct figure should beFig. S4f,\nnot Fig. S5f. We sincerely appreciate your careful attention to this detail. We have thoroughly\nreviewed and verified the references to all figures in the new manuscript to ensure their accuracy.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part\nof the Nature Communications initiative to facilitate training in peer review and to provide\nappropriate recognition for Early Career Researchers who co-review manuscripts.\nResponse: Thanks for your valuable comments on our manuscript.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nList of Changes\n1. As suggested by Reviewer 1, we have changed the description about the 1DPC in page 6.\n2. As suggested by Reviewer 1, we have revised the material designation from \"Si N \" to \"SiN \" in\n3 4 X\npage 6.\n3. As suggested by Reviewer 1, we have added the description of the PECVD(Plasma-Enhanced\nChemical Vapor Deposition) process in page 8.\n4. As suggested by Reviewer 1, we have swapped the labels ofFig. 1bandFig. 1cin the revised\nmanuscript.\n5. As suggested by Reviewer 1, we have added Fig. S6 to present the experimentally measured\ntransmittance curve of the 1DPC.\n6. As suggested by Reviewer 1, we have added scale bars toFig. 2a, 2b, 2d, and 2e.\n7. As suggested by Reviewer 1, we have added an explanation for the deviation from the ideal\nGaussian fitting in page 13.\n8. As suggested by Reviewer 2, we have added a discussion on the use of a smartphone and a small\nlens as the imaging system for the Chip-PSM in page 19 and Supplementary Section 9.\n9. As suggested by Reviewer 2, we have added details of the LED light source, including its source\nand model number, total power, and power density, in page 20. Additionally, we have\nsupplemented Supplementary Section 3 to explain the coupling efficiency of Chip-PSM.\n10.As suggested by Reviewer 2,we have added a comparison between prism-based PSM and Chip-\nPSM in page 11.\n11.As suggested by Reviewer 2, We have added detailed information about image processing in\nSupplementary Section 6.\n12.As suggested by Reviewer 2,we have updated Fig. S5f to Fig. S4f in page 17.\n13.We have added the number of measured nanoparticles in Fig.3 and included a note on\nmeasurement reproducibility in the figure caption.\n14.We have added National Nature Science Foundation of China (grant nos. 42375124, 52373122) to\nthe acknowledgments.\n15.We have added two corresponding authors, Gang Zou and Xu Liu, and supplemented the author\ncontributions.\n16.We rearranged the numbers of Supplementary Sections and Figures changed by the newly added\nsections and figures both in Main Text and SI.\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA\nA Point-by-point response to the referees’ comments\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nAuthors have answered appropriately the questions and therefore I believe the manuscript can be accepted..\nResponse: Thank you for your kind comment.\nReviewer #2 (Remarks to the Author):\nThe revision has addressed all my comments and suggestions\nResponse: Thank you for your kind comment.\nReviewer #3 (Remarks to the Author):\nI co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts.\nResponse: Thanks for your valuable comments on our manuscript.\nA list of changes\n1. We revised and shortened the abstract to approximately 150 words.\n2. We removed the exaggerated word ‘extremely’ on lines 218, 225 and 425.\n3. We used italics to denote scalar variables such as , , and , while vector variables were shown in boldface\nwithout italics.\n𝑘 λ θ\nUNIVERSITY OF SCIENCE & TECHNOLOGY OF CHINA"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Authors have answered appropriately the questions and therefore I believe the manuscript can be accepted.", "reviewer_2": "The revision has addressed all my comments and suggestions.", "reviewer_3": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature\nCommunications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career\nResearchers who co-review manuscripts."}, "author_response": "Response: Thank you for your kind comment.\nResponse: Thank you for your kind comment.\nResponse: Thanks for your valuable comments on our manuscript."}]

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