Design and synthesis of boron complexes as new Raman reporter molecules for sensitive SERS nanotags

Article abstract of DOI:10.1002/jrs.6020

A new series of boron complexes consisting of pyridine-pyrazole ligands has been designed, synthesized and evaluated for its Raman activity using solid-state Raman spectroscopy. The detailed surface-enhanced Raman scattering (SERS) study of selected dye 2

Full text of DOI:10.1002/jrs.6020

Received: 17 June 2020
DOI: 10.1002/jrs.6020
Revised: 9 October 2020
Accepted: 12 October 2020
R E S E A R C H A R T I C L E
Design and synthesis of boron complexes as new Raman
reporter molecules for sensitive SERS nanotags
Rashid Javaid1
| Nima Sayyadi1,2
| Kausala Mylvaganam1
|
Koushik Venkatesan1
| Yuling Wang1 | Alison Rodger1
1Department of Molecular Sciences,
Macquarie University, Sydney, New South
Wales, Australia
2ARC Centre of Excellence for Nanoscale
Bio photonics (CNBP), Macquarie
University, Sydney, New South Wales,
Australia
Abstract
A new series of boron complexes consisting of pyridine-pyrazole ligands has
been designed, synthesized and evaluated for its Raman activity using solid-
state Raman spectroscopy. The detailed surface-enhanced Raman scattering
(SERS) study of selected dye 2-(3-(pyridin-4-yl)-1H-pyrazol-5-yl) pyridine (P4)
on gold nanoparticles (AuNPs) of size 40 nm revealed that it can detect as
small as the nanomole level with signal quality that is superior to structurally
similar commercially available reporter molecules (RMs), rendering it a suit-
able RM for sensitive SERS nanotags.
Correspondence
Alison Rodger, Department of Molecular
Sciences, Macquarie University, Sydney
2109, NSW, Australia.
Email: alison.rodger@mq.edu.au
K E Y W O R D S
Funding information
boron complexes, molecular design, pyridine, Raman reporter, SERS
Macquarie University, Grant/Award
Number: iMQRES award scholarship
1 | INTRODUCTION
chemical enhancement of the signal is the result of
charge transfer between the molecule and support sur-
Surface-enhanced Raman scattering (SERS) has been
explored extensively during the last decade due to its
potential as an alternative to commonly used fluores-
cence methods for biological sensing.[1–3] Attractive fea-
tures of SERS include high sensitivity, excellent
photostability (no photobleaching), use of a single laser
source, multiplexing capability and high signal-to-noise
ratio. Another added advantage is that red to near-
infrared (NIR) laser wavelengths, which minimizes the
problem of interference due to auto fluorescence arising
from cells and tissues, can be used for SERS.[4] SERS
relies on the inelastic interaction of a photon with an
analyte. The inherent weakness of the Raman signal
intensity is enhanced using a metallic surface.[5] Electro-
magnetic enhancement of the SERS signal arises due to
excited electrons of metallic nanoparticles (NPs) oscillat-
ing in resonance with the incident optical field making a
localized surface plasmon resonance (LSPR).[6] Whereas,
face. SERS nanotags are commonly composed of a noble
roughed metallic support surface (usually gold or silver)
and an organic dye reporter molecule (RM).[4,7] Another
important component of a successful nanotag is the
aggregating agent, generally an inorganic salt, which
reduces the Columbic repulsion energy between the RM
and the metallic surface, allowing their tight bonding for
molecular finger printing.[8] An ideal RM usually has a
high Raman scattering cross section, a small number of
atoms, molecular symmetry, signal stability and a
surface-seeking group. It has been documented recently
that the introduction of hetero atoms into the RM facili-
tates the tight binding to the surface and excludes the
requirement of an aggregating agent.[9] Another impor-
tant aspect is the introduction of a second functional
group, such as carboxylic acid for direct bioconjugation.
Even though the RM is an important part of a
nanotag, its role has not been explored as
J Raman Spectrosc. 2020;1–8.
wileyonlinelibrary.com/journal/jrs
© 2020 John Wiley & Sons, Ltd.
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JAVAID ET AL.
comprehensively that of the metallic NPs. However,
recently various classes of organic compounds have been
rationally designed and tested for their usefulness as new
RMs. Graham and co-workers, for example, introduced
benzotriazole derivatives for use as dyes in surface-
enhanced resonance scattering with silver colloids.[10]
Maiti et al.[11] have reported cyanine and triphenylmeth-
ane covalently bound to the gold surface. Lambert and
co-workers designed and synthesized new RMs based on
alkyne moieties and used them for tissue imaging.[12]
Highly sensitive probes based on chalcogenopyrylium
were published by Harmsen et al.[13] The rational design,
supported by density functional theory (DFT) calcula-
tions of rhodamine with thiol groups at the xanthene
ring, has been reported by Brem and Schlücker.[14] The
above work highlights the importance of the design and
synthesis of new RMs with desired structural features
and unique fingerprints for selected analytical
applications.
2.2 | Methods
The progress of each reaction was monitored by thin
layer chromatography (TLC) analysis. High-resolution
mass spectra (HRMS) were recorded with an Agilent
6538 Q-TOF with dual electrospray ionization
(Supporting Information) source. 1H and 13C nuclear
magnetic resonance (NMR) spectra were recorded on a
Bruker Avance spectrometer 400 (1H) and 100 MHz
(13C) in CDCl3 (de-acidified by passing it through cal-
cium carbonate before use) and with dimethyl sulfox-
ide (DMSO). NMR and HRMS data are supplied as
Supporting Information. Ultraviolet (UV)–visible
absorption spectra were recorded on an Agilent Cary
Eclipse UV–visible spectrophotometer.
A
portable
Raman microscope (IM-52 Snowy Range, 407 S. 2nd
Street Laramie, WY 82070) with 785-nm laser excita-
tion and laser power of 100 mW was used for the
Raman analyses.
Boron complexes constitute versatile fluorescent dyes,
and their distinct properties include ease of synthesis,
structural modification, stability and compatibility with
the biological system as probes. The configuration of this
class can readily be controlled through the tuning of the
ligands. The scarcity of examples of the boron complexes
as RMs for SERS nanotags is mainly due to their low
aqueous solubility and lower affinity for the metal sur-
face. However, pyridine, the first molecule that was used
for the discovery of SERS,[15] and the recent reports on
the potential and applications of the boron
dipyrromethene compounds (BODIPYs) as SERS nan-
otags inspired us to design and synthesize a new class of
boron complexes based on pyridine-pyrazole ligands,and
further test their normal Raman spectra and the potential
as RMs for sensitive SERS nanotags.[16–19]
2.3 | Synthesis
Methyl picolinate used for the synthesis of 1,3-diketones
was synthesized according to a reported method.[20] The
synthesis and characterization of the 1,3-diketones:
1-phenyl-3-(pyridine-2-yl)
propane-1,3-dione
(B1),
1-(4-bromophenyl)-3-(pyridine-2-yl) propane-1,3-dione
(B2), 1-(4-methoxyphenyl)-3-(pyridine-2-yl) propane-1,-
3-dione (B3) and 1-(pyridine-2-yl)-3-(pyridine-4-yl)
propane-1,3-dione (B4); the pyridine-pyrazole ligands:
2-(3-phenyl-1H-pyrazol-5-yl)
pyridine
(C1),
2-(3-(4-bromophenyl)-1H-pyrazol-5-yl) pyridine (C2),
2-(3-(4-methoxyphenyl)-1H-pyrazol-5-yl) pyridine (C3)
and 2-(3-(pyridin-4-yl)-1H-pyrazol-5-yl) pyridine (C4);
and the pyridine-pyrazole boron complexes: 2-(5-phenyl-
1H-pyrazol-3-yl)
pyridine
boron
complex
(P1),
2 | EXPERIMENTAL
2.1 | Materials
2-(3-(4-bromophenyl)-1H-pyrazol-5-yl) pyridine boron
complex (P2), 2-(3-(4-methoxyphenyl)-1H-pyrazol-5-yl)
pyridine boron complex (P3), and 2-(3-(pyridin-4-yl)-1H-
pyrazol-5-yl) pyridine boron complex (P4) are given in
the Supporting Information.
All chemicals were purchased from Sigma-Aldrich,
Australia unless otherwise stated. BODIPY FL NHS (BDP
FL NHS) ester was purchased from Lumiphore. Water
was obtained from a Milli-Q plus system (Millipore Co.),
and resistivity was reported to be ≥18 MΩ.cm. Unless
otherwise noted, all chemicals were used without further
purification. The solvent tetrahydrofuran (THF) used for
the synthesis of 1,3-diketones was first dried with a
molecular sieve (4 Å) and was stored under an argon
atmosphere for 72 h before using. All reactions were car-
ried out under an inert atmosphere of argon.
2.4 | Normal Raman spectra of boron
complexes in solid state
To measure Raman spectra of a compound, it was gro-
und to fine powder, a thin layer of which was spread
evenly over a microscope slide, and the spectrum col-
lected with an integration time of 0.5
10 accumulations.
s
and

JAVAID ET AL.
3
2.5 | Adsorption of boron complex on
2.8 | Computational method
AuNPs
Gaussian-16 analytical software[21] was used for the
quantum chemical DFT calculations. The pyridine deriv-
ative (P4) structure was fully optimized using Becke three
hybrid exchange and the Lee–Yang–Parr correlation
functional[22,23] B3LYP and a Pople split valence diffused
and polarized 6-311++G(d,p) basis set. The vibrational
frequencies and the Raman activities were calculated on
the optimized structure. Vibrational band assignments
were made using the Gauss view 6.0.16 molecular visuali-
zation programme.[24] All calculations were carried out
in the gas phase.
P4 (Mol. Wt. 387.17 g/mol) (0.1 mg, 0.26 × 10−3 M) was
dissolved in DMSO (1 mL). The resulting stock solution
was used for serial dilutions, using 10 μL of the stock
solution and diluting it with DMSO (90 μL) and using the
second dilution as stock for the next and so on till
(0.26 × 10−9 M) was reached. Each solution was poured
into a separate vial with 1 mL of AuNPs (see Supportign
Information for the preparation method). The vials were
rotated overnight using a rotator operating at 300 rpm to
mix the two components. The solutions were then cen-
trifuged at 7,000 rpm for 5 min, and the supernatant was
discarded carefully without disturbing the bottom AuNPs
layer to remove excess P4 molecules. The residue was
suspended in Milli-Q water (1 mL) and was shaken well
and sonicated then centrifuged under same conditions.
The same process was repeated three times to remove
DMSO to avoid signals from the solvent. Finally, boron
complexes with AuNPs were obtained, and the adsorp-
tion was confirmed by measuring the UV–vis absorption
spectrum of every resulting solution.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization
Previous reports have established that pyridine-
pyrazolate ligands can be easily synthesized and tuned
with a tendency to bind with boron. Based on previously
documented literature methods, synthesis of pyridyl pyr-
azolate boron complexes was carried out via 1,3-diketone
pathways (Figure 1).
2.6 | Preparation of the standard Raman
reporter solution
Various ligands were synthesized using protocols
reported in the literature. Selected ketones were treated
with an ester in an inert atmosphere using sodium
hydride (NaH) in dry THF. The products of the reactions
were purified by column chromatography on silica gel
and suitable combination of organic solvents as eluents.
1H NMR studies revealed that the products existed in
keto-enolic tautomeric forms, with the enolic form being
the dominant. 1,3-Diketones were used to prepare the
pyridine-pyrazolate ligands by reacting them with excess
hydrazine. These ligands were, in turn, mixed without
further purification with BPh3 in toluene resulting in the
boron complexes. All the final complexes were character-
ized by NMR (1H and 13C) and HRMS (see Supporting
Information). All the synthesized boron complexes are
new except P1, which was synthesized according to a
reported method.[25]
2.6.1 | Boron dipyrromethene FL NHS
(BDP FL NHS) solutions
BDP FL NHS boron complex (Mol. Wt. 389.16 g/mol)
(0.1 mg, 0.26 × 10−3 M) was dissolved in Milli-Q water
(1 mL). Serial dilution was performed to get the desired
concentration and each one adsorbed on AuNPs, as
described above.
2.6.2 | Pyridine solutions
Pyridine (Mol. Wt. 79.10 g/mol) was prepared (0.2 mg,
0.26 × 10−3 M) in Milli-Q water (10 mL). Serial dilution
was performed to get the desired concentration and each
one adsorbed on AuNPs as described above.
3.2 | Raman spectroscopy (solid-state
experimental, DFT based and aqueous
solution)
2.7 | SERS analysis
The Raman spectra of the boron complexes and standards
were recorded in aqueous solution, with a portable Raman
microscope(IM-52SnowyRange)with785-nm laserexcita-
tion and the laser power of 100 mW. The spectra were col-
lectedatanintegrationtimeof0.5sand10accumulations.
A preliminary investigation of the Raman activity of the
synthesized boron complexes was done utilizing normal
Raman spectroscopy in the solid state. The Raman pro-
files of all compounds were quite similar due to the simi-
larity in the compounds' molecular structures (P1–P4). All

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JAVAID ET AL.
FIGURE 1 Schematic
route for the synthesis of
pyridine-pyrazolate boron
complexes
the compounds along with their Raman spectra, DFT cal-
culations and DFT(B3LYP)/6-31++G(d,p) optimized
structure are depicted in Figure 2a,b, respectively.
P1, P2 and P4 exhibited strong Raman signatures. A
possible reason for P3's poor performance could be the
presence of a p-OMe group that favours strong fluores-
cence thus depressing the Raman signals. Although the
Raman signal of P3 was weak, the vibrational peaks at
1,000, 1,380 and 1,610 cm−1 were still observable, which
were attributed to the vibrational modes of aromatic
3.3 | SERS analysis
AuNPs of size 40 nm were adopted in this work. The
selected dye P4 (0.26 × 10−3 M) was mixed with
AuNPs freshly prepared as described in the Supporting
Information. The mixture was shaken well overnight
allowing the maximum adsorption of P4 to occur
through chemisorption and physisorption. Strong bond-
ing between the RM and support surface was through
the nitrogen of the pyridine side chain resulting in N–
AuNP bonding as well as electrostatic interactions due
to partial positive charges on the pyridine nitrogen
and partial negative charge on boron as a result of
dative bond formation. Although the nitrogen atoms in
the pyrazole provide a potential binding site for AuNPs
besides pyridine, they are sterically hindered due to
presence of bulky phenyl groups (Figure 2b). More-
over, molecular rearrangements during adsorption pro-
cess cannot be ignored. More conclusive evidence
came from the shifting of distinctive peaks
1,610–1,593 cm−1 arising from C N stretching while
N–N stretching (1,380 cm−1) was unchanged after
adsorption endorsing the fact that the pyridine is par-
ʋ
C–H, ʋN–N and ʋC N, respectively.
Among all the tested compounds, the pyridine deriva-
tive (P4) exhibited the most intense Raman spectral pro-
file. The major Raman peaks obtained, along with peak
assignments, are presented in Table 1. Given that P4
exhibited well-resolved signals and structural features
including nitrogen for attachment to the metal surface, it
was chosen for more detailed analysis. The solution
Raman spectrum of the bare RM (P4) was also recorded
in 1% DMSO/H2O, for comparison. Only weak Raman
signals were observed for P4 in solution (Figure 3b).
DFT calculations of gas-phase P4 (i.e., no inter-
molecular hydrogen bonding) were compared with the
experimental solid-state data. The DFT(B3LYP)/6-311+
+G(d,p) calculated frequencies only differ slightly from
these experimentally determined values. The C N
stretch, which is mixed with C C, C–C and C–N
stretches, showed a weak peak in the calculated Raman
spectrum and a strong one in the experiment. However,
strong calculated peaks are apparent at slightly higher
energy (1,633, 1,634 and 1,637 cm−1) corresponding to a
mix of C–C and C C stretches.
ticipating
in
electron
sharing
(Supporting
Information—Figure A2.37). It retained its properties
even after exposure to a laboratory environment for a
few weeks. It is important to note that no aggregating
agent was needed for the adsorption of the RM to the
surface. The confirmation of adsorption was made
through the measurement of optical properties of the
AuNPs and AuNPs/RM solutions. The UV–visible
absorption of freshly prepared AuNP samples indicated

JAVAID ET AL.
5
FIGURE 2 (a) Structure and solid-state Raman spectra of compounds (P1–P4) with 785-nm laser excitation and laser power of 100 mW.
(b) The DFT(B3LYP)/6-31++G(d,p) optimized structure and the calculated Raman spectrum of P4
the distinct peak at 536 nm, assigned to the AuNPs
due to their LSPR. After the adsorption of the RM, a
red shift was observed with the peak value moving to
555 nm (Figure 3a). The typical cross section for
Raman scattering is 10−30 cm2/sr with surface-
enhanced Raman cross section about 10−26 cm2/sr as
reported for the typical Raman dye molecules
(e.g., rhodamine 6G and crystal violet).[26] Thus, the

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JAVAID ET AL.
TABLE 1 Chemical structure and major vibrational modes of P4 in solid state together with the corresponding DFT(B3LYP)/6-311++G
(d,p) calculated gas-phase values
Mode of vibration and relative Raman
intensity
Chemical structure
ʋ (cm−1) (experimental)
ʋ (cm−1) (calculated)
864, 866
860
Aromatic C–H wagging (w)
Aromatic ring breathing (s)
N–N stretching (s)
1,000
1,380
1,610
1,015, 1,016
1,345
1,580, 1,597
C N stretching (s)/(w)
Note: s = strong; w = weak.
FIGURE 3 (a) Absorption spectra of P4 adsorbed on AuNPs (red), AuNPs (black) and P4 (blue) in aqueous media. (b) Raman spectra of
bare P4 in 1% DMSO/H2O (red) and P4–Au (black) both at 0.26 × 10−3 M. (c) Concentration-dependent surface-enhanced Raman scattering
(SERS) spectra of P4 to determine the limit of detection. (d) SERS dilution study of P4 at 1,396 cm−1. Error bars represent a standard
deviation from three replicates and 10 scans of each with 785-nm laser excitation and exposure time of 0.5 s
Raman cross section of the new developed molecule
was estimated about 10−26 cm2/sr. The exact value
depends on the excitation frequency and the SERS
substrate. A comparison of SERS spectra for P4 in
aqueous media and with AuNPs demonstrates a signifi-
cant Raman signal enhancement (Figure 3b).

JAVAID ET AL.
7
evaluated using solid-state normal Raman spectroscopy.
The signals of the most Raman active dye were enhanced
using 40-nm AuNPs. The dye binding to the AuNPs was
strengthened by partial charges on the boron and nitrogen
atoms. This resulted in a tight association with the surface
and Raman signal enhancement. The SERS effect was
found to decrease linearly with concentration upon dilu-
tion rendering it useful for quantitative analysis. Further,
the LOD was calculated to be 0.26 nM at 1,396 cm−1. The
reported dye retained resolved sharper signals even at this
low concentration when compared with structurally simi-
lar commercial Raman reporters. The targeted synthesis
and ease of molecular tuning with high affinity for AuNPs
opens a new window for chemists to develop new RM pro-
bes for selected applications. In future, new classes of com-
pounds with distinct molecular signatures and the
capability of multiplexing by using secondary functional
groups will be synthesized for targeted biological
applications.
FIGURE 4 Comparison of surface-enhanced Raman
scattering (SERS) spectra of P4 (black) with BODIPY BDP FL NHS
(red) and pyridine (blue) all measured at 2.6 × 10−8 M
3.4 | Sensitivity study of the SERS
nanotag
A concentration study of P4 RM was carried out to deter-
mine the limit of detection (LOD, Figure 3c). The sharp
peak at 1,396 cm−1 was used to calculate the LOD to be
0.26 nM (Figure 3d) by calculating three times the stan-
dard deviation of the blank divided by the gradient of the
straight line in Figure 3d.
ACKNOWLEDGMENTS
This work was funded by the University of Macquarie
including a Higher Degree Research Scholarship for RJ.
ORCID
Rashid Javaid https://orcid.org/0000-0003-2532-6835
Nima Sayyadi https://orcid.org/0000-0002-0044-2170
Kausala Mylvaganam https://orcid.org/0000-0002-
5226-2833
3.5 | Comparison with commercial
nanotags
Koushik Venkatesan https://orcid.org/0000-0002-3046-
2017
Given that P4 exhibited a sharp Raman signature even at
low concentration, it was interesting to compare it with
structurally similar and commercially available tag mole-
cules. BODIPY FL NHS (BDP FL NHS) ester was chosen
due to structural similarity as both form a five-membered
ring complex with boron, which attached via two nitro-
gen atoms. Pyridine was chosen as a conventional second
standard because P4 contains pyridine, which is assumed
to be a key part of its interaction with the AuNPs. Even
at 2.6 × 10−8 M, P4 retained a sharp molecular fingerprint
whereas the others exhibit broader bands (Figure 4). All
had similar intensities.
Yuling Wang https://orcid.org/0000-0003-3627-7397
Alison Rodger https://orcid.org/0000-0002-7111-3024
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How to cite this article: Javaid R, Sayyadi N,
Mylvaganam K, Venkatesan K, Wang Y, Rodger A.
Design and synthesis of boron complexes as new
Raman reporter molecules for sensitive SERS
nanotags. J Raman Spectrosc. 2020;1–8. https://doi.
org/10.1002/jrs.6020