# Broadband plasmonic nanoparticles: fabrication, optical properties, and
implications in liquid light chemiluminescence enhancement

Daler R. Dadadzhanov  ITMO University, 49 Kronverksky Ave., 197101, St.
Petersburg, Russia<EMAIL_ADDRESS>Tigran A. Vartanyan  ITMO
University, 49 Kronverksky Ave., 197101, St. Petersburg, Russia  Peter S.
Parfenov  ITMO University, 49 Kronverksky Ave., 197101, St. Petersburg, Russia
Mikhail A. Baranov  ITMO University, 49 Kronverksky Ave., 197101, St.
Petersburg, Russia  Alina Karabchevsky  Electrooptics and Photonics
Engineering Department, Ben-Gurion University, Beer-Sheva 8410501, Israel
<EMAIL_ADDRESS>
###### Abstract

Chemiphores are entities, which exhibit wide-band light emission without any
external light source but just due to the chemical reaction resulting in the
chemiluminescence effect. Since the chemiphores usually have low quantum
efficiency, chemiluminescence is a weak optical effect. We found that
plasmonic nanoparticles can efficiently enhance the peculiar effect of
chemiluminescence due to the acceleration of the radiative decay of the
chemiphore excited state which, in turn, enlarges the chemiluminescence yield.
Correspondingly, plasmonic nanoparticles are nanoparticles with sub-wavelength
sizes experiencing the absorption band in specific wavelength which are
characterized by unique optical properties, as well as high localization of
electromagnetic radiation. However, the broadband properties of plasmonic
nanoparticles and their implications in liquid light, the chemiluminescence
effect, is overlooked. Therefore, they can attract attention as novel
materials for photonics, sensing, and forensic science. Here, fabrication
techniques of broadband plasmonic nanoparticles are reported, and their
interesting optical properties together with their applications in
chemiluminescence effect are discussed, as well. We fabricated the
nanoparticles with laser ablation in liquids (LAL) technique and propose the
physical vapor deposition (PVD) synthesis with annealing-assisted treatment
for further studies. Both techniques are accessible and allow production of
ensembles of nanoparticles having shape and size distributions to exhibit
broad plasmonic resonance which fit the wide-band emission of a chemiphore.
Our results, in particular, a specific design for plasmonic nanoparticles
placed on the dielectric material, lead the way toward a new generation of
chemiluminescence-based devices starting from sensing, healthcare, biomedical
research and quantum systems such as pump-free laser sources.

###### keywords:

plasmonic nanoparticles chemiluminescence microfluidic chiplabel-free sensor
metal-enhanced chemiluminescence luminnol

Electrooptics and Photonics Engineering Department, Ben-Gurion University,
Beer-Sheva 8410501, Israel Ilse Katz Institute for Nanoscale Science &
Technology, Ben-Gurion University, Beer-Sheva 8410501, Israel Ilse Katz
Institute for Nanoscale Science & Technology, Ben-Gurion University, Beer-
Sheva 8410501, Israel

Chemiluminescence, or liquid light effect, is an emission of photons resulting
from an exothermic reaction 1. This fascinating optical effect that finds its
use in various applications: from forensic science targeted to detection the
blood traces at crime scene Barni et al. 2, to industrial bio-chemistry 3, 4.
Due to the emerging need of securing societies, ensuring the safety of the
citizens and protection of freedom a development of the technology to
strengthen forensic science has been recently boosted. Luminol is a chemiphore
(chemical that exhibits chemiluminescence) which under certain conditions
emits blue-glow light 5. Forensic investigators use Luminol to detect trace
amounts of blood at crime scenes since it reacts with the iron in hemoglobin;
biologists use it in cellular assays to detect copper, iron and specific
proteins 6. However, the quantum efficiency of Luminol is very low, which
significantly limits its potential applications.

One of the experimental tools for observing the chemiluminescence effect is a
microfluidic chip, which is employed for the efficient study of low volume
samples by isolating key phenomena from influential surrounding environment 7.
The mixing during the flow-injection in microfluidic chips helps in obtaining
the efficient chemical reaction in which the chemiphores and their oxidants
are mixed to emit light. Crucial developments for the fabrication approaches
for the microfluidic technology platform already gave birth to emerging
applications such as the lung-on-a-chip platform 8, 9, 10, single-cell
functional proteomics 11, 12, and measuring the dynamics of green fluorescence
protein 13, 14. However, the efficient enhancement of poor quantum emitters,
such as the chemiphore Luminol, has never been proposed. This efficient
emission of Luminol is expected to find a purpose in forensic science-on-a-
chip, such as an accurate DNA profiling with active lasing dust.

In this paper we report on the repertoire of efficient Luminol enhancement
possibilities within frontiers of nanotechnology and quantum mechanics while
enhancing the interaction of plasmon-Luminol due to the fabrication-tuned the
wide-band plasmonic resonant absorption of the nanoantennas. It is important
to note that he quantum efficiency of the chemiluminescence process is quite
low due to the competition with a multitude of non-radiative decay processes
15. Indeed, radiative decay acceleration of the chemiphore excited state is a
reliable way to enhance the chemiluminescence yield that attracts attention of
many scientists 1, 16, 17. The surface plasmon resonance in metallic
nanoparticles 18, 19 can be employed for the acceleration of these radiative
transitions. To maximize of the chemiphores-nanoparticles interaction, metal
nanoparticles have to be placed at an optimum distance from the chemiphores
20. The shape and material of the particles have to be accurately chosen to
ensure the overlapping of their plasmonic band with the emission band of the
chemiphores. Several studies have been performed to investigate the use of
plasmons for enhancing the radiative decay in a microfluidic chip 21; however,
those researchers used non-resonant nanoparticles. New proof of concept
experiment was carried out by using a microfluidic chip 1, wherein we utilized
the commercially available nanoparticles with non-optimized spectral location
of the plasmon absorption band. We attribute the possibility of enhanced
chemiluminescence to the Purcell effect. However, the chemiluminescence is
limited in efficiency because excited molecules can lose their energy through
non-radiative processes, such as internal conversion and intersystem crossing.
An excited molecule can decay either radiatively at the rate $\gamma_{R}$ or
non-radiatively at the rate $\gamma_{NR}$. Therefore, the chemiluminescence
enhancement with these nanoparticles is not sufficient and alternative
fabrication techniques are needed to engineer the wide-band and tunable
plasmonic resonances for the efficient interaction between the nanoparticle
acceptors and the chemiphore donors.

Overall, for effective interaction, between the plasmonic nanoparticle and
chemiphore, two conditions should be met: 1) the plasmon band has to overlap
in spectrum with the chemiluminescence emission band 22 and 2) chemiphore
molecules are to be brought in the close proximity of metal nanoparticles 23.
In this work, we explore the possibility of fulfilling both conditions with
silver nanoparticles as acceptors and the chemiphore molecules as donors of
energy quanta. As the plasmon band of a silver nanosphere lies on the far blue
wing of the Luminol chemiluminescence band, we employed two different
techniques to shift the plasmon band in the desired red direction: enlarging
of the nanosphere radius and changing of the nanoparticle shape. To keep the
transfer of the technology feasible, we rely on the well - established and
rather inexpensive techniques of silver nanoparticle production, namely,
pulsed laser ablation in liquids 24, 25, 26 and physical vapor deposition on a
dielectric substrate. Our proposals are supported by numerical simulations
that showed that the plasmon band of silver nanoparticle may be shifted up to
optimally overlap with the Luminol chemiluminescence band.

Our results can provide a gain media, utilizing plasmonic enhancement, as a
toolkit for Forensic laboratories. Gain medium or active medium, is capable of
amplification of light by utilizing the process of stimulated emission.
Carefully designed nanostructures are equipped of both enhancing fundamental
interactions and directing signals to an observer or camera. This engineered
chemiluminescence-based devices with a gain media will improve detection
limits of chemiluminescence for forensic science, facilitate the imaging, and
confer new insights in DNA profiling through important physical effects such
as amplification process involved in stimulated emission.

The utilization of chemicals such as ’light source’ is one of the major
challenges facing photonics today. The revolutionizing nature of our study is
in the efficient interaction between Luminol and nanostructures. Outcomes of
our research make it possible to take a step forward and increase the accuracy
of DNA profiling, increase the efficiency of chemically induced photon
emission by developing new engineered nanostructures capable of efficient
chemiluminescence and construct a general platform for lasing dust.

##  1 Materials and methods

###  1.1 Synthesis of colloidal silver nanoparticles

Silver nanoparticles have been synthesized using the laser ablation approach.
The experimental setup is shown in Fig.1a. The silver plate (99,99$\%$ purity)
has been used as a target for the synthesis. Laser ablation was caused by
pulsed Nd:YAG solid-state laser (SOLAR Laser Systems). The laser source
enables the second harmonic with 532 nm and turned in the Q-switching regime
during ablation process. The laser beam, with a diameter of 5 mm, was focused
using a lens with focal length of 60 mm through a transparent liquid
(deionized water) on the surface of the metal plate. The surface of the liquid
remained free, and the thickness of its layer above the surface of the plate
was several centimeters. To get broad size distribution of silver
nanoparticles the target was irradiated for 15 min. The pulse energy was set
to 100 mJ. The pulse duration and the repetition rate were, respectively, 10
ns and 5 Hz, respectively.

###  1.2 Characterization

The morphology studies of the sample with silver nanoparticles synthesized
using pulsed laser ablation were performed using an atomic force scanning
microscope on Solver PRO-M (NT-MDT) using silicon probes NSG01. Microscopic
examination was performed in the tapping (semi-contact) mode. The resonance
frequency and curvature radius of the probe in the tapping mode were 283 kHz
and 10 nm, respectively. Scanning electron microscope images of supported
silver nanoparticles were obtained by means of MERLIN (Carl Zeiss) microscope
at 15 kV. Extinction spectra of the colloidal solution in a cuvette with the
thickness of 1 cm were collected using an SF-56 spectrophotometer (LOMO) with
1 nm resolution in the range of 250 - 800 nm at room temperature. The
chemiluminescent spectra of Luminol were recorded by a charge-coupled device
(CCD), Lumenera Infinity 2-3C (Lumenera Corporation, 7 Capella Crt. Ottawa,
Ontario, Canada).

###  1.3 Numerical simulation

The Finite-Element-Method (FEM) implemented in COMSOL Multiphysics 5.4
software was used to solve Maxwell’s equations for stationary problems. We
calculated absorption, scattering and extinction cross-sections spectra of
silver nanoparticles of different shapes and on different substrates. We
included the perfectly matched layer (PML) in the form of a physical sphere
domain to absorb all scattered light. We define the thickness of PML as the
half of the incident wavelength. The incident field oscillated in the
x-direction (parallel to the substrate) with a k-vector in the -z direction
(perpendicular to the substrate). The absorption cross section
($\sigma_{abs}$) was calculated by integrating the power loss density over the
particle volume. The scattering cross-section ($\sigma_{sc}$) was derived by
integrating the Poynting vector over an imaginary sphere around the particle.
The total extinction cross-section was calculated as the sum of the both
$\sigma_{ext}$ = $\sigma_{abs}$ \+ $\sigma_{sc}$. The complex refractive index
of the silver nanoparticles was obtained from the experimental work of Johnson
and Christy 27

##  2 Results and Discussion

###  2.1 Experimental and numerical studies for nanoparticle-Luminol system

Figure 1: (a) Experimental set up of pulsed laser ablation of silver target in
water. The dimensions are not to scale.(b) Scanning electron microscope image
of silver nanoparticles on the carbon grid. The scale bar is 200 nm. (c) The
size distribution of silver nanoparticles dav = 24.6 nm.(d) 3D atomic force
microscopy image of silver nanoparticles dried on the quartz substrate
obtained in the tapping mode regime. (e) atomic force microscopy image (the
top view) with size scale bar from 15 to 65 nm. (f) The cross-section profile
of fabricated silver nanoparticles. The height of nanoparticles varied from 10
to 60 nm.

We propose the plasmonic system design for tuning the broad absorption of
plasmonic nanoparticles for efficient enhancement of the chemiluminescence
effect. To prove the efficiency of the proposed nanoparticle \+ chemiphore
system, we first constructed the experimental setup shown in Fig.1a and
fabricated silver nanoparticles in liquid. During the experiment, performed at
the room temperature, the silver target was ablated by the laser for different
exposure times. This is done to find the time dependence on the particle size
which, in turn, gives the broad enough plasmonic band to enhance the
chemiluminescence effect and using the distilled water without a stabilizing
agents or surfactants. After fabrication, we characterized the nanoparticles
with the atomic force microscope (AFM) and the scanning electron microscope
(SEM). Based on the SEM (Fig.1b) micrographs, we estimated the size
distribution of nanoparticles. The histogram in Fig.1c confirms that the
colloidal solution contains fabricated nanoparticles with a broad size
distribution. The average diameter size of silver nanoparticles is 24.6 nm.
The measured extinction spectra of colloidal silver nanoparticles exhibit the
pronounced plasmon band in spectral range of 300 to 600 nm. Dried silver
nanoparticles were imaged by AFM on a quartz substrate (Fig.1d,e). According
to AFM data of the cross-section (Fig.1f), the particle height for a specific
region in AFM image varied from h=10 to h=60 nm (taking into account the shape
of the AFM probe), while their dimensions in substrate plane were from 10 to
50 nm in diameter.

Fig.2a shows a photograph of the beaker with fabricated silver nanoparticle in
distilled water. We notice that the liquid has definite yellow color. This
color can be explained by the spectral location of absorption band of
fabricated nanoparticles in the absorption spectra, as shown in Fig2a. Thus,
the laser ablation of the silver target lasted 15 minutes leads to the
formation of the silver nanoparticles with the broad plasmon band centered at
400 nm.

Figure 2: (a) Measured in visible spectrum optical densities of colloidal
silver nanoparticles prepared by laser ablation technique in water and
normalized to the maximum value. In the inset is the colloidal solution with
distinctive yellow color. (b) Overlap between the measured chemiluminescence
intensity of the Luminol solution (green line) and the calculated extinction
spectra of supported silver nanoparticle submerged into Luminol solution;
extinction cross-section spectra of silver nanoparticle with the height of 27
nm for the enhancement of the Luminol chemiluminescence band at 452 nm and
with the height of 20 nm for the Luminol band at 489 nm. The diameters of
silver nanoparticles in both cases are 40 nm. (c) Extinction cross-section
spectra of silver nanoparticles with the heights of 35 nm and 26 nm on the
magnesium fluoride substrate. The diameters in both cases are equal to 52 nm.
The 3D models of silver nanoparticles placed on the respective substrates and
submerged into Luminol solution are illustrated in the inserts.

For this reason, we addressed the deposition of silver nanoparticle on the
dielectric substrate which is more favorable for further plasmon shifting. A
much larger long-wavelength shift of the silver nanoparticles plasmon bands
can be obtained by physical vapor deposition (PVD) of silver on a transparent
dielectric substrate, for example, sapphire, magnesium fluoride (MgF2), quartz
(SiO2) or titanium dioxide (TiO2). Fig.2b,c show the numerical calculations of
extinction, absorption and scattering cross-sections spectra for nanoparticles
on different substrates. During the modeling, the silver nanoparticle was
embedded in a Luminol solution and placed on the two types of dielectric
material. We show that by changing the size parameter we can tune the optical
properties of the silver nanoparticle. Firstly, to find the optimized
parameter we measured the chemiluminescent emission spectra of Luminol in
water oxidized by hydrogen peroxide. Since the Luminol was diluted with high
ratio in water, for calculation, we took for calculation the refractive index
of the surrounded media of 1.33. According to our calculations presented in
Fig.2b, ideal matching is achieved for silver nanoparticles with the height
(denoted as c in Fig.2) of 20 nm and 29 nm deposited on a quartz substrate
(the refractive index is 1.46 28). In this case the diameter was constant (40
nm). However, the calculations show that the plasmon band of hemispherical
silver nanoparticles on a quartz substrate cannot be brought in an exact
resonance with the second Luminol chemiluminescence emission peak located at
452 nm. Indeed, even in the hemispheres with the diameter as small as 5 nm
plasmonic peak is located at 472 nm. To overcome this difficulty, the
substrate of smaller refractive index may be used. Of particular interest is
using of a wide-spread material MgF2, which has the refractive index as low as
1.38 29. In Fig.2c, we show the extinction cross-section spectra of silver
nanoparticles with diameters of 52 nm and two different heights: h=35 nm and
h=26 nm on the magnesium fluoride substrate. Fig. 2c shows that the suitable
overlapping between the Luminol emission band and the silver nanoparticle
plasmon band is achieved when: a) the hemispherical particles height of h=26
nm is placed on top of the magnesium fluoride substrate and b) the height of
the nanoparticle is enlarged up to h=35 nm. We notice that the sapphire
substrate having the refractive index as high as 1.77 30, 31 is not suitable
for efficient overlapping of Luminol emission, since the long-wavelength shift
is too large even for the very small silver hemispheres.

Fig. 3 summarizes the schematics of the chemiluminescence enhancement
mechanism. Fig.3a represents the Luminol excitation in the course of a
chemical reaction with oxidizer followed by two competitive decay routes:
radiative and non-radiative. The effect of silver nanoparticles on the Luminol
emission is depicted in Fig.3c. The expected chemiluminescence enhancement can
be explained by coupled emission from excited states of Luminol and plasmon
mode in silver nanoparticles 21.

Figure 3: Schematics of the enhancement mechanism of chemiluminescence effect.
(a) Oxidized Luminol lead to competitive process of radiative decay (emission
of the photon) and non-radiative decay (heat). The blue color emission is
attributed to emission of Luminol. (b) The proposed mechanism metal-enhanced
chemiluminescence in the presence of silver nanoparticle. (c) The general
Jablonski energy level diagram modified for the chemiluminescence emission
with the presence of silver nanoparticle 32. The chemiluminescence emission
origin as result of chemical oxidation process. The molecular transition from
the excited (S1) to the electron ground state(S0), emitting a photon and
heat.$\gamma_{R}$ and $\gamma_{NR}$ are the radiative and non-radiative decay
of Luminol molecule. $\gamma_{R^{\prime}}$ is the radiative decay rate with
silver nanoparticle.

Indeed, as it was reported in previous works 33, 34, in contrast of the metal-
enhanced fluorescence (MEF) 35, 36 where the molecule initially must be pumped
by external light, metal-enhancement chemiluminescence (MEC) origins stem by a
chemical oxidation process as shown in the Jablonski diagram (Fig.3c).
Therefore, the decay rate can be increased by plasmon nanoparticle.

We experimentally prove the concept of operation and feasibility of the
engineered optimized system. First, we fabricated the silver nanoparticles on
the quartz substrate using the physical vapor deposition. Silver target was
sputtered in high vacuum at the rate of 0.1 Å/s and then annealed at 200o C.
Second, we controlled the spectral position of the plasmon absorption band by
the amount of the sputtered metal measured as an equivalent thickness of the
continuous film. We found that the maximum of the plasmon absorption band is
located at 450 nm when the equivalent thickness is 3 nm. However, the plasmon
band shifts to 488 nm when the equivalent thickness is 9 nm 37, 38 according
to the requirements of the effective acceleration of radiative transitions in
Luminol. We conclude that the fabrication of silver nanoparticles using the
physical vapor deposition technique in vacuum followed by the self-
organization of nanoparticles is an affordable and convenient method that
potentially can lead to plasmonic enhancement for the Luminol
chemiluminescence.

In the above discussion we were mainly concerned with the spectral overlap of
the plasmon and chemiluminescence bands. Now we address the second requirement
of placing the chemiphore molecules in the close proximity of metal
nanoparticles. One can underline that the arrangement of particles on the
transparent dielectric substrate makes it possible to control the distance
between plasmonic nanoparticles and chemiphores using, for example, a polymer
thin layer as a spacer. Another advantage is the possibility of incorporating
the substrates modified with plasmonic nanoparticles in a microfluidic chip
for chemiluminescence enhancement. The width as well as the position of
plasmon bands of nanoparticles obtained via PVD and subsequent annealing can
be readily tuned to match the position and the width of the chemiphore bands.
The outcomes of our research leading to the chemiluminescence enhancement are
very important in chemi- and bio- luminescence effects for variety of
applications in biology, sensing, healthcare and forensic science.

###  2.2 Numerical calculation for alternative MEC systems on dielectric
substrate

As mentioned before, in the present work, we consider the chemiphore Luminol,
however here we show designs for the enhanced chemiluminescence of several
well-known chemiphores. On the other hand, emission of different chemiphores
may be enhanced via the suggested technique and the dedicated choices of
substrates and annealing procedures as well 37. We found a number of
substrate-nanoparticle pairs optimized for surface enhanced chemiluminescence
of several well-known chemiphore: KMnO4 in Fig.4, Lucigenin in Fig.5 a,b, Ce
(IV) in Fig.5 c,d, and bis-(2,4,6-tricholorophenyl) oxalate (TCPO) in Fig.5
e,f.

Figure 4: Calculated absorption, scattering and extinction cross-section
spectra of the silver nanoparticle. The silver nanoparticle deposited on the
TiO2 substrate. The diameter of hemisphere is d=17.5 nm. Figure 5: Calculated
absorption, scattering and extinction cross-section spectra of the silver
nanoparticle: (a)deposited on the SiO2 substrate for Lucigenin. The diameter
of hemisphere is d=28 nm. (b) the silver nanoparticle on the MgF2 substrate
with diameter of 49 nm for Lucigenin system; (c) deposited on the SiO2
substrate for Ce (IV). The diameter of hemisphere is d=90 nm. (d) the silver
nanoparticle on the Al2O3 substrate (d=53.5 nm); (e)deposited on the SiO2
substrate for TCPO system. The diameter of hemisphere is 32 nm. (f) the silver
nanoparticle on the MgF2 substrate with diameter of 47 nm for overlapping with
TCPO chemiphore.

We optimized the cross-section spectra of the silver hemisphere for KMnO4
chemiluminescent emission spectra (Fig.4). The maximum emission peak is 700 nm
39. An alternative example of a chemiluminescent agent we considered
Lucigenin. This chemiphor is widely used in analytical science and has a
maximum emission peak of 480 nm. 40, 41, 42, 43 (Fig.5a,b). The cross-section
spectra of silver hemisphere optimized for Cerium (IV) chemiluminescent
emission spectra presented in Fig.5c,d. The maximum emission peak 550 nm 44,
45. Next, we calculated the suitable parameters for the plasmon nanoparticle
on the top of dielectric substrate for TCPO chemiluminescence emission
spectra. The maximum emission peak is 485(Fig.5e,f) nm 46.

##  3 Conclusions

In summary, we proposed revolutionizing new approaches in designing new
materials assembled from nanoparticles and chemiphores for enhancing effects
such as of chemiluminescence as well as spasing for emerging applications such
as DNA profiling and forensic studies - all those, hold a promise to deliver
substantial enhancement in specific and accurate detection, which is not
possible via traditional approaches. We noticed that the red edge of the
plasmon band located at 400 nm overlaps with the Luminol emission bands. We
found that the arrangement of particles on the surface of transparent
dielectrics makes it possible to control the distance between the light-
emitting species and nanoantennas to increase the Purcell effect of the
materials include arrays of sub-wavelength particles, which vary in their
shape and size. Combining these materials with recent advances in
chemiluminescence enhancement effect demonstrated using flow-injection systems
opens the door for new chemiphore-based light sources. These light sources in
which the poor quantum emitter, chemiphore, is replaced by high quantum
efficiency of plasmonic nanoantenna could not have been achieved by
conventional manufacturing processes in which particles size and shape are
fixed.

## Acknowledgement

The work was supported by the State of Israel- Innovation Authority, Ministry
of Economy Grant No. 62045; the Ministry of Science and Higher Education of
Russian Federation (Project 3.4903.2017/6.7). This work also was financially
supported by Government of Russian Federation, Grant 08-08. The research
described was performed as part of a joint Ph.D. program between the BGU and
ITMO.

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