rus
Issue #6/2021
I.V.Yaminskiy, O.V.Sinitsyna, A.I.Akhmetova, S.A.Senotrusova, А.А.Piryazev, Е.P.Kozhina, S.А.Bedin
Use of microlenses to improve the optical microscopy resolution and enhance Raman scattering
Use of microlenses to improve the optical microscopy resolution and enhance Raman scattering
DOI: 10.22184/1993-8578.2021.14.6.382.388
When interacting with light the microspheres form a narrow beam called a photonic nanojet. The use of this effect allows of implementing a super high-resolution microscopy to exceed the diffraction limit and the Raman scattering signal amplification. To enhance the extremely weak RS-signal, it is necessary to use the multiplicative surfaces. Here we demonstrate the photonic nanojet effects used for RS-signal enhancement in combination with giant Raman scattering produced by the active substrates, silver nanowires, nanoparticles and barium titanate microspheres.
When interacting with light the microspheres form a narrow beam called a photonic nanojet. The use of this effect allows of implementing a super high-resolution microscopy to exceed the diffraction limit and the Raman scattering signal amplification. To enhance the extremely weak RS-signal, it is necessary to use the multiplicative surfaces. Here we demonstrate the photonic nanojet effects used for RS-signal enhancement in combination with giant Raman scattering produced by the active substrates, silver nanowires, nanoparticles and barium titanate microspheres.
Теги: microlens photonic nanojet raman scattering микролинзы нанопроволоки наночастицы оптическая микроскопия фотонная наноструя
USE OF MICROLENSES TO IMPROVE THE OPTICAL MICROSCOPY RESOLUTION AND ENHANCE RAMAN SCATTERING
When interacting with light the microspheres form a narrow beam called a photonic nanojet. The use of this effect allows of implementing a super high-resolution microscopy to exceed the diffraction limit. The second important application of the photonic nanojet is the Raman scattering (RS) signal amplification. However, to enhance the extremely weak RS-signal, it is necessary, as a rule, to use the multiplicative surfaces. In this work we demonstrate the photonic nanojet effects used for RS-signal enhancement in combination with giant Raman scattering produced by the active substrates with the ensembles of silver nanowires and nanoparticles together with the microsphers made of barium titanate.
INTRODUCTION
Intense development of nanotechnologies requires the improvement of visualization methods and super-high resolution control. In 2011, it was proposed to use microspheres [1] to obtain super-high resolution in optical microscopy, which made it possible to observe elements of ~ 50 nm scale. Microspheres are placed on the sample surface. Light, passing through the microsphere dia. 1–100 µm, is focused in a narrow beam and forms a photonic nanojet (PNJ). The enlarged virtual image of the sample area under the PNS illumination can be observed with a common optical microscope.
Combination of the optical microscopy and Raman scattering spectroscopy (RSS) makes it possible to identify the chemical composition of samples and study the crystal lattice peculiarities to increase a research information capacity. Intensity of RS may be insufficient to register a spectrum in a nanoparticle study. The use of multiplicative surfaces and additional focusing makes it possible to inhance the RS-signal sufficiently [2].
Use of the nanostructured surfaces in a study of various biological and chemical objects allows not only of increasing the RS-signal, but also of conducting measurements at low power of laser emission within a few seconds. It can be applied to diagnose nanoconcentrations of substances, including living organisms. Enhancement of the RS-signal using the multiplicative surfaces can be explained by areas with high-intensive local electric fields, so-called “hot spots”. The microlenses are helpful in the living organism diagnostics. Generally, silicon plates were used as objects. For example, a scientific team from Croatia studied geometric aspects of RS-signal amplification by silicon microspheres [3]. The study was conducted in three stages: characterization of an incident beam using the knife-edge method horizontal and, for the first time, vertical Raman reflection, and ray transfer matrix analysis. A distinct area of RS-signal amplification from a silicon plate generated by the PNJ microsphere was observed. The amplification was the highest when the incident beam size corresponded to the diameter of microsphere, and the beam focus was below the top of the sphere.
In this work the method of combined microlens microscopy and RSS was applied to study silver nanowires and nanoparticles. These objects are very interesting when preparing substrates intended for the surface enhanced Raman spectroscopy (SERS) to detect trace quantities of the chemical substance.
RESEARCH METHODS AND MATERIALS
Substrates with silver nanowire ensembles
In this work, metal nanowires (NP) were created by template synthesis method in the pores of polymer track membranes (TM) made of polyethylene terephthalate (prepared in Flerov Laboratory of Nuclear Reactions, JINR, Dubna), by the template synthesis method were created.
The pore diameter in the used TM is 100 nm, and the surface density is 9.3 · 108 cm–2. Pores in the initial TM were filled with silver by the electrochemical method with the formation of the massive metal substrate made of copper [4, 5]. Silver NP diameter corresponded to the pore diameter of the original template (100 nm), and the length was determined by the time of the template filling and equaled 10 µm. The polymer template with NW inside was washed from electrolyte and dissolved in a concentrated alkali solution (6M NaOH) at 60 °С. After dissolving the template, copper substrates with silver NW were washed in distilled water and dried. When drying, the characteristic bands of agglomerated NWs were formed under the action of capillary forces. Rhodamine 6G (R6G) was chosen as the studied substance to obtain SERS-spectra on the substrates with silver 100 nm NWs. It is an organic dye with the well-studied spectrum, so it is possible to accurately compare the obtained results with the data presented by other scientific teams. Collecting of rhodamine 6G SERS-spectra (concentration is 100 µg/ml) adsorbed on the placed vertically massive silver NWs was performed with a Horiba LabRam Evolution Raman spectrometer. The spectra were measured using a 532 nm wavelength laser with a power of 1 mW. The exposure time was equal to 4 s for two collecting cycles. The laser spot diameter was 10 µm when using a 50x lens. The optical microscope receiving the signal was equipped with a motorized stand with automated focus adjustment.
Besides, the spectrometer was equipped with two diffraction gratings, a high-efficient detector, a cooled Peltier element and a set of neutral gray filters to adjust the emission power on the sample. After preparing of the substrates, they were certified by the scanning electron microscopy methods.
Figure 1 presents the micrograph of the substrate with the ensemble of silver NWs.
Preparation of the samples with silver nanoparticles
Studied were the samples of mica with silver nanoparticles prepared by the following method: aqueous solution of silver nanoparticles (50 mg/l) was applied onto the mica plate in 5 layers with prolongation. Figure 2 demonstrates the image of nanopatricles obtained by atomic force microscopy (AFM) and the distribution of particles by sizes.
To measure parameters of the samples with microlens microscopy and RSS, the surfaces were coated with BaTiO3 microspheres with a size of 30 to 100 µm and density of 4.22 g/cm3, the refraction index 1.9.
The laser beam was passed in the lens for focusing on a sample. The generated signal was collected by a microsphere and further studied with an optical microscope. In this case, a microsphere provides magnification of a local field.
RESULTS AND DISCUSSION
Silver nanowires (NWs)
Figure 3 shows the optical image of the silver NWs array surface. The microsphere dia. 35 µm placed in the image centre and its focus must be located at a distance of ~ 18.4 µм from the bottom sphere edge, according to [7]. The enlarged image can be seen through the microlens, where the separate tips of silver NWs are visible. According to the electron microscopy data, the diameter of NWs is 100 nm.
Therefore, a microlens allows of obtaining the optical image behind the diffraction limit, which limits the resolution of standard optical microscopy at a level of 200 nm. A microlens forms a virtual image with magnification ~ 15, which is captured by the microscope lens.
Then, the spectra of 10 µg/l Rhodamine 6G were obtained on these substrates without lenses and with their use (see Fig.4). The characteristic peaks of this substance are well visible in the image of the spectrum.
The intensity of molecular SERS-signal without microlenses is greater than with their use. It can be explained because during the microlens use the spectrum of smaller area is collected, hence, a smaller number of molecules took part in this process.
The density of NWs on the substrate is 1.2 · 109 cm–2. It is clear, that at the SERS-spectra registration without lenses, when a laser beam spot is 10 µm, the NWs number is, approximately, ~ 942. The number of NWs when using a lens is, approximately, ~ 60. In conclusion, use of microlenses makes it possible to study the spectra of a small number of molecules, which is several orders of magnitude less than without the use of microlens.
Silver nanoparticles
Silver nanoparticles are too small for their visualization with microlens microscopy, because an average particle height, in accordance with AFM, is 15 nm (see Fig.2). But it is an example of RS-signal magnification with microlens dia. 13.5 µm (see Fig.5). Figure 5a shows the RS spectra obtained from the various points on the surface, inside the lens and outside of it (coloured in Fig.5b with black and red lines, correspondingly). The peak in the region of 240 cm–1 corresponds to the Ag–O stretching vibrations in the nanoparticle surfaces [8]. Figure 5c demonstrates how the signal from this peak increases in the microlens centre at magnification of ~ 3. Therefore, the use of microlenses in optical microscopy and RSS allows of obtaining the better optical resolution and increases the RSS sensitivity. Microlens microscopy makes it possible to study the spectra of substances down to the single molecules. The following development of microlens microscopy is connected with manufacturing of the films with microspheres [9] and microlens scanning systems to enlarge a field of view.
ACKNOWLEDGEMENTS
The study was completed with the financial support of the RFBR and the London Royal Society No. 21-58-10005 and Ministry of Science and Higher Education of the Russian Federation. The measurements of RS were completed with the financial support of the Institute of Problems of Chemical Physics of RAS state task 0074-2019-0014 (state registration number is АА-19-119101590029-0).
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
When interacting with light the microspheres form a narrow beam called a photonic nanojet. The use of this effect allows of implementing a super high-resolution microscopy to exceed the diffraction limit. The second important application of the photonic nanojet is the Raman scattering (RS) signal amplification. However, to enhance the extremely weak RS-signal, it is necessary, as a rule, to use the multiplicative surfaces. In this work we demonstrate the photonic nanojet effects used for RS-signal enhancement in combination with giant Raman scattering produced by the active substrates with the ensembles of silver nanowires and nanoparticles together with the microsphers made of barium titanate.
INTRODUCTION
Intense development of nanotechnologies requires the improvement of visualization methods and super-high resolution control. In 2011, it was proposed to use microspheres [1] to obtain super-high resolution in optical microscopy, which made it possible to observe elements of ~ 50 nm scale. Microspheres are placed on the sample surface. Light, passing through the microsphere dia. 1–100 µm, is focused in a narrow beam and forms a photonic nanojet (PNJ). The enlarged virtual image of the sample area under the PNS illumination can be observed with a common optical microscope.
Combination of the optical microscopy and Raman scattering spectroscopy (RSS) makes it possible to identify the chemical composition of samples and study the crystal lattice peculiarities to increase a research information capacity. Intensity of RS may be insufficient to register a spectrum in a nanoparticle study. The use of multiplicative surfaces and additional focusing makes it possible to inhance the RS-signal sufficiently [2].
Use of the nanostructured surfaces in a study of various biological and chemical objects allows not only of increasing the RS-signal, but also of conducting measurements at low power of laser emission within a few seconds. It can be applied to diagnose nanoconcentrations of substances, including living organisms. Enhancement of the RS-signal using the multiplicative surfaces can be explained by areas with high-intensive local electric fields, so-called “hot spots”. The microlenses are helpful in the living organism diagnostics. Generally, silicon plates were used as objects. For example, a scientific team from Croatia studied geometric aspects of RS-signal amplification by silicon microspheres [3]. The study was conducted in three stages: characterization of an incident beam using the knife-edge method horizontal and, for the first time, vertical Raman reflection, and ray transfer matrix analysis. A distinct area of RS-signal amplification from a silicon plate generated by the PNJ microsphere was observed. The amplification was the highest when the incident beam size corresponded to the diameter of microsphere, and the beam focus was below the top of the sphere.
In this work the method of combined microlens microscopy and RSS was applied to study silver nanowires and nanoparticles. These objects are very interesting when preparing substrates intended for the surface enhanced Raman spectroscopy (SERS) to detect trace quantities of the chemical substance.
RESEARCH METHODS AND MATERIALS
Substrates with silver nanowire ensembles
In this work, metal nanowires (NP) were created by template synthesis method in the pores of polymer track membranes (TM) made of polyethylene terephthalate (prepared in Flerov Laboratory of Nuclear Reactions, JINR, Dubna), by the template synthesis method were created.
The pore diameter in the used TM is 100 nm, and the surface density is 9.3 · 108 cm–2. Pores in the initial TM were filled with silver by the electrochemical method with the formation of the massive metal substrate made of copper [4, 5]. Silver NP diameter corresponded to the pore diameter of the original template (100 nm), and the length was determined by the time of the template filling and equaled 10 µm. The polymer template with NW inside was washed from electrolyte and dissolved in a concentrated alkali solution (6M NaOH) at 60 °С. After dissolving the template, copper substrates with silver NW were washed in distilled water and dried. When drying, the characteristic bands of agglomerated NWs were formed under the action of capillary forces. Rhodamine 6G (R6G) was chosen as the studied substance to obtain SERS-spectra on the substrates with silver 100 nm NWs. It is an organic dye with the well-studied spectrum, so it is possible to accurately compare the obtained results with the data presented by other scientific teams. Collecting of rhodamine 6G SERS-spectra (concentration is 100 µg/ml) adsorbed on the placed vertically massive silver NWs was performed with a Horiba LabRam Evolution Raman spectrometer. The spectra were measured using a 532 nm wavelength laser with a power of 1 mW. The exposure time was equal to 4 s for two collecting cycles. The laser spot diameter was 10 µm when using a 50x lens. The optical microscope receiving the signal was equipped with a motorized stand with automated focus adjustment.
Besides, the spectrometer was equipped with two diffraction gratings, a high-efficient detector, a cooled Peltier element and a set of neutral gray filters to adjust the emission power on the sample. After preparing of the substrates, they were certified by the scanning electron microscopy methods.
Figure 1 presents the micrograph of the substrate with the ensemble of silver NWs.
Preparation of the samples with silver nanoparticles
Studied were the samples of mica with silver nanoparticles prepared by the following method: aqueous solution of silver nanoparticles (50 mg/l) was applied onto the mica plate in 5 layers with prolongation. Figure 2 demonstrates the image of nanopatricles obtained by atomic force microscopy (AFM) and the distribution of particles by sizes.
To measure parameters of the samples with microlens microscopy and RSS, the surfaces were coated with BaTiO3 microspheres with a size of 30 to 100 µm and density of 4.22 g/cm3, the refraction index 1.9.
The laser beam was passed in the lens for focusing on a sample. The generated signal was collected by a microsphere and further studied with an optical microscope. In this case, a microsphere provides magnification of a local field.
RESULTS AND DISCUSSION
Silver nanowires (NWs)
Figure 3 shows the optical image of the silver NWs array surface. The microsphere dia. 35 µm placed in the image centre and its focus must be located at a distance of ~ 18.4 µм from the bottom sphere edge, according to [7]. The enlarged image can be seen through the microlens, where the separate tips of silver NWs are visible. According to the electron microscopy data, the diameter of NWs is 100 nm.
Therefore, a microlens allows of obtaining the optical image behind the diffraction limit, which limits the resolution of standard optical microscopy at a level of 200 nm. A microlens forms a virtual image with magnification ~ 15, which is captured by the microscope lens.
Then, the spectra of 10 µg/l Rhodamine 6G were obtained on these substrates without lenses and with their use (see Fig.4). The characteristic peaks of this substance are well visible in the image of the spectrum.
The intensity of molecular SERS-signal without microlenses is greater than with their use. It can be explained because during the microlens use the spectrum of smaller area is collected, hence, a smaller number of molecules took part in this process.
The density of NWs on the substrate is 1.2 · 109 cm–2. It is clear, that at the SERS-spectra registration without lenses, when a laser beam spot is 10 µm, the NWs number is, approximately, ~ 942. The number of NWs when using a lens is, approximately, ~ 60. In conclusion, use of microlenses makes it possible to study the spectra of a small number of molecules, which is several orders of magnitude less than without the use of microlens.
Silver nanoparticles
Silver nanoparticles are too small for their visualization with microlens microscopy, because an average particle height, in accordance with AFM, is 15 nm (see Fig.2). But it is an example of RS-signal magnification with microlens dia. 13.5 µm (see Fig.5). Figure 5a shows the RS spectra obtained from the various points on the surface, inside the lens and outside of it (coloured in Fig.5b with black and red lines, correspondingly). The peak in the region of 240 cm–1 corresponds to the Ag–O stretching vibrations in the nanoparticle surfaces [8]. Figure 5c demonstrates how the signal from this peak increases in the microlens centre at magnification of ~ 3. Therefore, the use of microlenses in optical microscopy and RSS allows of obtaining the better optical resolution and increases the RSS sensitivity. Microlens microscopy makes it possible to study the spectra of substances down to the single molecules. The following development of microlens microscopy is connected with manufacturing of the films with microspheres [9] and microlens scanning systems to enlarge a field of view.
ACKNOWLEDGEMENTS
The study was completed with the financial support of the RFBR and the London Royal Society No. 21-58-10005 and Ministry of Science and Higher Education of the Russian Federation. The measurements of RS were completed with the financial support of the Institute of Problems of Chemical Physics of RAS state task 0074-2019-0014 (state registration number is АА-19-119101590029-0).
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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