Quantification of methylene blue aggregation on a fused silica ...

Abstract
Quanti®cation <strong>of</strong> <strong>methylene</strong> <strong>blue</strong> <strong>aggregation</strong> on a fused
silica surface and resolution <strong>of</strong> individual
absorbance spectra
Shane M. Ohline * , Sunyoung Lee, Stacie Williams, Connie Chang
Department <strong>of</strong> Chemistry, Wellesley College, Wellesley, MA 02481, USA
Received 11 July 2001
The absorbance spectra <strong>of</strong> individual <strong>methylene</strong> <strong>blue</strong> aggregates adsorbed by spin coating on a fused silica surface
are resolved. Using singular value decomposition and a model incorporating Beer's Law and elemental conservation, a
varied concentration family <strong>of</strong> UV±Vis spectra are ®t. Results indicate that H-dimer formation occurs beginning at a
surface concentration <strong>of</strong> 3 10 13 molecules=cm 2 , and further aggregate formation, including J-type dimers, is signi®cant
above 2 10 14 molecules=cm 2 . Based on our results, past assumptions that a spin-coated surface using a millimolar
coating solution) results in a monolayer <strong>of</strong> <strong>methylene</strong> <strong>blue</strong> monomers should be reexamined. Ó 2001 Elsevier
Science B.V. All rights reserved.
1. Introduction
28 September 2001
Chemical Physics Letters 346 2001) 9±15
Past work to examine the orientation <strong>of</strong> large
dye molecules, including <strong>methylene</strong> <strong>blue</strong> and
rhodamine dyes, adsorbed on fused silica surfaces
[1±7] has used a variety <strong>of</strong> adsorption methods to
produce a thin ®lm <strong>of</strong> dye at the surface. The
methods used have included spin coating [2], deposition
from a concentrated solution [3], and
wiping the surface with lens paper and a drop <strong>of</strong>
dye solution [8]. Once coated, various nonlinear
[2,3] and linear [7] spectroscopic techniques were
used to determine the orientation <strong>of</strong> the dye molecules
relative to surface normal. Despite extensive
* Corresponding author. Present address: Department <strong>of</strong>
Chemistry, 1253 University <strong>of</strong> Oregon, Eugene, OR97403-
1253, USA. Fax: +1-781-283-3642.
E-mail address: sohline@wellesley.edu S.M. Ohline).
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 0 1 ) 0 0 962-9
www.elsevier.com/locate/cplett
research by others on <strong>aggregation</strong> <strong>of</strong> dyes in solution
[9±14], much <strong>of</strong> this interfacial work paid
only cursory attention to the possibility <strong>of</strong> dimer
or other aggregate formation on the surface [2,5,7].
In fact, much <strong>of</strong> the interpretation <strong>of</strong> the dye/
surface spectroscopic data relied on the absence <strong>of</strong>
aggregates. For example, in second harmonic
generation SHG) studies, to simplify data analysis,
the direction <strong>of</strong> the electronic transition dipole
is frequently used as an indication <strong>of</strong> the dominant
tensor element contributing to the surface nonlinear
susceptibility. Subsequently, this is used to
determine the orientation <strong>of</strong> the molecule relative
to surface normal [2,3]. If dimers or other aggregates)
are present on the surface, the direction <strong>of</strong>
the electronic transition dipole is not always
known or obvious. There is no reason to assume it
will lie along the same axis in the monomer and the
dimer. This is particularly true if one obtains SHG

10 S.M. Ohline et al. / Chemical Physics Letters 346 2001) 9±15
using a wavelength resonant with an electronic
transition <strong>of</strong> an aggregate. Thus formation <strong>of</strong> aggregates
on the surface can dramatically a€ect the
®nal results one obtains in these orientation determinations.
Most recently, researchers hoping to increase
the e€ectiveness <strong>of</strong> photovoltaic cells have investigated
dye <strong>aggregation</strong>. To expand the range <strong>of</strong>
the electromagnetic spectrum over which solar
cells can collect energy, dye molecules have been
applied to nanostructured semiconductor surfaces
[15,16] and semiconductor nanoparticles [16±19] to
determine if the dyes can e ciently transfer excitation
energy to the semiconductor. Information
which provided spectra and concentrations <strong>of</strong> the
various aggregates would be useful for these researchers
in designing suitable coatings for
photovoltaic cells.
One problem inherent to most previous studies
involving dyes is a lack <strong>of</strong> quantitative information
concerning the concentration <strong>of</strong> the various aggregate
species on solid surfaces. Without a simple
relationship between the concentrations <strong>of</strong> the
monomers and dimers e.g., an equilibrium constant),
as one has in solution, it is di cult to determine
analytically the relative concentration <strong>of</strong>
each aggregate on the surface and their spectral
signature. Using singular value decomposition
SVD), Beer's Law and elemental conservation, we
provide quantitative information about the degree
<strong>of</strong> <strong>aggregation</strong> <strong>of</strong> <strong>methylene</strong> <strong>blue</strong> on a quartz
surface and identify the species involved. Brand et
al. [20] have used a similar method to resolve the
spectra <strong>of</strong> rhodamine 6G aggregates up to the
tetramer) in an aqueous solution. Using a spin
coating technique, we show that formation <strong>of</strong> Htype
dimers occurs even at 4 10 5 M coating
solution concentrations, and more complex spectra
are observed between 2 10 4 and 5 10 3 M.
These concentrations are within the range used by
others in their SHG work typically 1 10 3 M)
[2,3,8]. Finally, we analyze spectra on the surface
up to 5:5 10 15 molecules=cm 2 coating solution
5 10 3 M), which show both <strong>blue</strong> and red-shifted
absorption peaks relative to the monomer, indicating
both H- and J-type dimers are present.
Using this novel technique to determine quantitatively
aggregate concentrations on a surface, we
show that past assumptions regarding surface
coating could be revisited.
2. Experimental
Solutions <strong>of</strong> <strong>methylene</strong> <strong>blue</strong> trihydrate Sigma,
MB-1) were prepared in 200 pro<strong>of</strong> ethanol. The
concentrations Ctot) ranged from 4:16 10 5 M
0.5%) to 5:20 10 3 M and were made by serial
dilutions from a stock solution. This range <strong>of</strong>
concentrations was chosen to match those used by
others in their orientation studies <strong>of</strong> dyes on surfaces
[2,3]. Flat fused silica windows 1 in. diameter,
3/16 in. thick, ESCO Products P110188) were
cleaned in boiling sulfuric acid for 5 min, rinsed
with deionized water then dried in an oven for
several hours. The windows were then coated with
the ethanolic solutions by spin coating. A
2 ll 2:5% drop <strong>of</strong> the dye solution was placed in
the center <strong>of</strong> each quartz window; these were then
placed inside a hanging bucket centrifuge Beckman
GS-6R) equipped with ¯at bottomed buckets
and were spun at 2000 rpm for 2 min until the
ethanol had completely evaporated. This left a
measured circle <strong>of</strong> ca. 13 mm 1 mm) diameter <strong>of</strong>
dye coating the surface. Using the area <strong>of</strong> the dye
circle and the number <strong>of</strong> <strong>methylene</strong> <strong>blue</strong> molecules
in the 2 ll drop, surface concentration in molecules/cm
2 , C S ) was calculated …C S ˆ Ctot
2 ll=area†. A UV±Vis spectrum was then obtained
<strong>of</strong> each surface Varian, Cary 500 NIRUV±Vis
Spectrometer) using a solid sample holder. An
uncoated quartz window spectrum was used as a
background and was subtracted from each dye
coated window. The sample was scanned at 1 nm/s
from 400 to 800 nm, averaging 20 points per nanometer.
A linear ®t to the baseline was performed
and subtracted. The data were smoothed once
before analysis using a binomial smoothing algorithm.
3. Results and analysis
Experimental data for a series <strong>of</strong> solutions
ranging between surface concentration <strong>of</strong>
4:7 10 12 ±6:6 10 14 molecules=cm 2 are shown in

Fig. 1. Corresponding solution concentrations are
given in Table 1. The changes in the spectra, even
at these low optical densities are clearly shown.
The peak maximum around 650 nm has long been
attributed to monomer absorbance [9], while the
peak at 600 nm grows in at higher concentrations.
Using exciton splitting theory [1,17,21], this <strong>blue</strong>
shifted peak is attributed to a sandwich-type dimer
H-type) in which the angle between the planes <strong>of</strong>
<strong>methylene</strong> <strong>blue</strong> are between 54.7° and 90°. Redshifted
peaks, relative to the monomer, are attributed
to transitions from the ground state to a
lower-lying S 1 state allowed only in J-type dimers
Fig. 1. Experimental data for <strong>methylene</strong> <strong>blue</strong> on quartz. See
Table 1 for concentrations corresponding to the labels.
which have an angle between the methlyene <strong>blue</strong>
planes <strong>of</strong> less than 54.7° [1,17,21]. Oblique dimers
with an angle greater than 90° between the
monomers), result in both red and <strong>blue</strong> shifted
peaks.
To identify the aggregates and their relative
concentrations, we follow and modify the method
<strong>of</strong> Brand et al. [20]. First, we use the method <strong>of</strong>
SVD [22] to determine the number <strong>of</strong> spectrally
independent species involved in the total spectrum.
SVD <strong>of</strong> our 6 370 matrix 6 spectra each at a
di€erent concentration, 370 wavelengths in the
431±800 nm range) gave the results shown in Fig. 2.
Table 1
Solution concentrations used to coat quartz surfaces and resulting surface concentration C S ) shown in Fig. 1
Spectrum Solution concentration
M) 0.5%)
S.M. Ohline et al. / Chemical Physics Letters 346 2001) 9±15 11
Fig. 2. SVD results for the data presented in Fig. 1. The singular
values are noted by m and are normalized to the highest
value. Each graph has the same scale as that noted in the upper
left hand corner <strong>of</strong> this ®gure.
Surface concentration
mol/cm 2 )
Surface concentration
molecules/cm 2 )
a 5:20 10 4 1:1 :1 10 9 6:6 :9 10 14
b 2:60 10 4 2:6 :3 10 10 1:6 :1 10 14
c 1:04 10 4 1:0 :1 10 10 6:2 :6 10 13
d 4:16 10 5 4:7 :5 10 11 2:8 :3 10 13
e 2:08 10 5 2:4 :2 10 11 1:4 :1 10 13
f 5:20 10 6 7:8 :9 10 12 4:7 :5 10 12

12 S.M. Ohline et al. / Chemical Physics Letters 346 2001) 9±15
The singular value for each spectral vector is given
by the value m in each frame. The values are
normalized by the highest value for comparison.
From these spectral vectors, we see that two values
are clearly above the noise level in the spectra,
strongly suggesting the presence <strong>of</strong> two species,
most likely the monomer and the dimer. The third
spectral vector also may be signi®cant, possibly
indicating the presence <strong>of</strong> a trimer. SVD alone
does not resolve the spectral signature <strong>of</strong> each
aggregate. However, the spectra in Fig. 1 are a
linear combination <strong>of</strong> the spectra <strong>of</strong> each spectrally
distinct chemical species. To recover the
spectrum for each species, we use a model to ®t our
data. First, we use elemental conservation, the fact
that the total concentration in solution is the
weighted sum <strong>of</strong> each species
Ctot ˆ X
nCn; …1†
n
where Ctot is the total concentration <strong>of</strong> <strong>methylene</strong>
<strong>blue</strong> in the ethanolic solution before coating, n ˆ 1
for the monomer, n ˆ 2 for the dimer, etc., and Cn
is the concentration <strong>of</strong> each n-mer in the solution.
Second, Beer's law states that the absorbance,
A…k†, can be expressed in terms <strong>of</strong> the concentrations,
Cn, and the molar extinction coe cient <strong>of</strong>
each n-mer, n…k†
A…k† ˆL X
Cn n…k†; …2†
C S
tot
n
where L is the optical path length. The units <strong>of</strong>
L Cn can be expressed in moles/cm2 , which is
surface concentration CS ). Because we know only
the Ctot and CS tot , and not each Cn or CS n , and we do
not know the path length L) for our samples, we
use the surface concentration in Eqs. 1) and 2) to
obtain
X
ˆ …3†
n
n
nC S
n
and
A…k† ˆ X
C S
n n…k†: …4†
By ®tting our data to Eqs. 3) and 4) simultaneously,
we obtain values for n…k† for each species
and the surface concentration <strong>of</strong> each n-mer on the
surface. Before we perform the ®tting <strong>of</strong> the data
in Fig. 1, the experimental absorbance measurements
are normalized to account for the di€erences
in magnitude between the lowest and the highest
total surface concentration. We divide the absorbance
in each spectrum by the area under that
spectrum. In this way, we do not overemphasize
the highest concentrations in our ®tting procedure.
To perform our numerical ®t <strong>of</strong> the data set, we
constructed an algorithm using Matlab ver. 6.0,
The Mathworks), Eqs. 3) and 4) and a nonlinear
least squares Levenburg±Marquardt minimization
routine. The function minimized, v 2 , was de®ned
as the weighted sum <strong>of</strong> the squares <strong>of</strong> the di€erences
between the acquired data A…k† and C S tot )
and the ®tting parameters, as de®ned in Eqs. 3)
and 4)
v 2 ˆ XNsam XNsp mˆ1
nˆ1
…CS tot † m nCS h i2 n m
r 2 mn …CS tot † n
‡ XNsam
PNsp X A…k† m nˆ1
mˆ1 k
…CS n † h i2 m …k† nm †
r2 mkA…k† ;
m
…5†
where Nsam corresponds to the number <strong>of</strong> samples
from which we obtained spectra 6 for Fig. 1); Nsp
corresponds to the total number <strong>of</strong> n-mers on the
surface e.g., n ˆ 3 for a ®t <strong>of</strong> monomer, dimer and
trimer); k corresponds to the number <strong>of</strong> wavelengths
collected in each spectrum.
Variance in each data point are represented by
r2 mk and r2 mn . Variances are noted for the CS tot data
in Table 1. A typical value for r2 mk was 10 5 based
on repeat absorbance measurements <strong>of</strong> the same
surface. With typical absorbance readings on the
order <strong>of</strong> 0.001, the value for r2 mkA…k† m was on the
order <strong>of</strong> 10 8 . The importance in proper weighting
has been outlined elsewhere [23].
Because we do not know our CS n or n…k† for any
species, an in®nite variety <strong>of</strong> solutions are equivalent
[24]. One must place additional constraints
on the system to ®nd an unique solution. We place
constraints on our system based on: i) the results
from the SVD analysis that there are 2 or possibly
3 species), ii) the fact that Eq. 3) is valid, and
therefore poses a constraint on the system while
®tting spectral data, and iii) that all values <strong>of</strong> CS n
and n…k† be positive.

The results <strong>of</strong> our analysis <strong>of</strong> Fig. 1 are shown in
Fig. 3. From these results, we ®nd that only the
monomer and H-type dimer appear to be present up
to surface concentrations <strong>of</strong> 6:6 10 14
molecules=cm 2 . Although the spin-coating method
was employed here, we have used other techniques
to coat the surface, including solution dipping [3]
and application <strong>of</strong> solution by lens tissue [8] and ®nd
very similar spectral results to those observed in Fig.
1. This indicates that the methods <strong>of</strong> coating do not
generally a€ect the degree <strong>of</strong> <strong>aggregation</strong>, and our
method <strong>of</strong> analysis can be used for a variety <strong>of</strong>
coating techniques. However, spin coating enables a
non-spectroscopic determination <strong>of</strong> surface concentration,
necessary to complete this analysis.
Also, we note that the values we obtained for 1 650
nm) and 2 600 nm) are similar to those obtained for
methlyene <strong>blue</strong> in solution [14,25].
To determine if trimers were also present on the
surface, we ®t the data in Fig. 1 to a three species
model. We ®nd that although v 2 is lower for the
three species ®t, no unique spectral signature <strong>of</strong> the
trimer could be determined in the range <strong>of</strong> surface
concentrations shown in Fig. 1. The dominant
Fig. 3. Results obtained by ®tting experimental data in Fig. 1
to Eqs. 3) and 4) simultaneously. The top ®gure shows the
resolution <strong>of</strong> the absorption spectra <strong>of</strong> the monomer and Htype
dimer, while the bottom ®gure shows the quantitative
values for the surface concentrations <strong>of</strong> both the monomer and
dimer.
S.M. Ohline et al. / Chemical Physics Letters 346 2001) 9±15 13
trimer peak appeared at 650 nm, directly below the
monomer. Additionally, the calculated surface
concentrations for the trimer CS 3 ) were on the
order <strong>of</strong> four orders <strong>of</strong> magnitude smaller than
those <strong>of</strong> the monomer and dimer. Therefore, we
deduce that monomers and H-type dimers are the
primary species up to a surface concentration <strong>of</strong>
6:6 1014 molecules=cm2 see Table 2).
In another experiment, the results <strong>of</strong> which are
shown in Fig. 4, we increased surface concentration
up to 5:5 1015 molecules=cm2 . These spectra
are particularly interesting because, after clear <strong>blue</strong>
shifting <strong>of</strong> the peak from 656 to 600 nm, indicating
the formation <strong>of</strong> an H-type dimer, the 600 nm
peak continues to increase in magnitude and
broaden to the <strong>blue</strong>, and a new peak appears at 690
nm. Using the analysis outlined in detail above, we
®t these data to two, three and four species models.
Results for this set <strong>of</strong> spectra indicate a signi®cant
presence <strong>of</strong> the H-type dimer, but also a J-type
dimer beyond a coating concentration <strong>of</strong> 2 10 4
M. J-type dimers will appear to the red <strong>of</strong> the
monomer peak, as justi®ed by exciton splitting
theory [1,21]. Based on the statistics <strong>of</strong> this ®t, the
stoichiometric coe cient <strong>of</strong> both the 600 and 690
nm peak must be two, indicating that both are
dimers rather than higher aggregates. Work will
continue to characterize these species. Additionally,
¯uorescence spectroscopy <strong>of</strong> these samples
will assist in characterizing the aggregates, as an
H-type dimer does not ¯uoresce, while J- and oblique-type
dimers do.
4. Discussion and conclusion
Results from Experiment 1 alone shown in Fig.
1) indicate that between 1 10 4 and 1 10 3 M
Table 2
Results from ®tting <strong>of</strong> data from Fig. 1
Model, n v2 weighted)
1, 2 136.031
1, 3 136.069
1, 4 137.386
1, 2, 3 133.442
1, 2, 4 133.673
The ®nal analysis results are shown in Fig. 3.

14 S.M. Ohline et al. / Chemical Physics Letters 346 2001) 9±15
Fig. 4. Experimental results for <strong>methylene</strong> <strong>blue</strong> coated on a
fused silica surface. Corresponding surface concentrations are
given in Table 3.
coating solution, dimerization is signi®cant.
Therefore past SHG results should require the
molecules be modeled not as rod-like monomers
but also as dimers. Although, in their work on
<strong>methylene</strong> <strong>blue</strong> at the fused silica surface, Corn et
al. [3] determined two rather than one, as other
SHG analysis has used for large aromatic dye
molecules [2]) molecular tensor elements contributed
to the surface nonlinear susceptibility, this
work still assumed a monomer as the dominant
species on the surface. Several wavelengths were
used to obtain a SHG signal, two <strong>of</strong> these wavelengths
585 and 615 nm) overlap dimer rather
than monomer resonance, as is shown by our
spectral signature for the H-type dimer Fig. 3).
Thus contributions to the SHG signal would be
dominated by dimers rather than monomers.
Recent work by Leach et al. [4] evaluated <strong>aggregation</strong>
<strong>of</strong> similarly sized) rhodamine 6G on
fused silica and found dimers were present at
concentrations in the range used previously to
determine molecular orientation at this interface
[2]. They also found the SHG signal from this
surface oscillated. Each maximum in SHG signal
corresponded to the completion <strong>of</strong> a new monolayer
and apparent ordering <strong>of</strong> the probed surface
[4]. Leach's work proved SHG signal does not
arise solely from the single dye monolayer directly
adsorbed to the fused silica surface, but rather
from that surface and any ordered bulk above this
layer. Therefore contributions from ordered dimers,
at or in the bulk above the surface, will
contribute to SHG signal and a€ect orientation
determination). Unpublished atomic force images
from our lab indicate that both <strong>methylene</strong> <strong>blue</strong>
and rhodamine 6G coated fused silica surfaces
coated by all <strong>of</strong> the three methods described here
and a 1 10 3 M solution) are composed <strong>of</strong> ca. 30
nm thick islands separated by several hundred
nanometers [26]. These images, with the UV±Vis
spectra analyzed in this paper, indicate that these
coating methods do not result in a simple monolayer
<strong>of</strong> dye monomers. The surface created is
signi®cantly more complicated than was previously
assumed.
By showing the formation <strong>of</strong> H- and J-type
dimers at higher surface concentrations, we also
indicate that <strong>methylene</strong> <strong>blue</strong> may be <strong>of</strong> interest to
Table 3. Solution concentrations used to coat quartz surfaces and resulting surface concentration C S ) in mol/cm 2 shown in Fig. 4
Spectrum Solution concentration
M) 0.5%)
Surface concentration
mol/cm 2 )
Surface concentration
molecules/cm 2 )
a 5:20 10 3 9:2 1 10 9 5:5 :9 10 15
b 2:60 10 3 2:0 :2 10 9 1:2 :1 10 15
c 2:60 10 4 3:4 :4 10 10 2:0 :2 10 14
d 1:04 10 4 1:4 :1 10 10 8:1 :9 10 13
e 4:16 10 5 6:3 :6 10 11 3:8 :4 10 13
f 2:08 10 5 2:7 :3 10 11 1:6 :2 10 13
g 1:04 10 5 1:4 :1 10 11 8:1 :9 10 12

those considering it as an energy transferring dye
in photovoltaic cells. Because it has long been
believed that only H-type dimers, which do not
¯uoresce, were formed by <strong>methylene</strong> <strong>blue</strong>, this dye
was not generally useful to transfer energy to an
underlying semiconductor. However, J-type dimers,
represented by the peak at 690 nm may be
more useful to transfer energy to a semiconductor
surface. Finally, this new numerical method <strong>of</strong>
determination <strong>of</strong> aggregate spectra and concentrations
on a surface should prove useful for a
number <strong>of</strong> dye/surface combinations, as quantitative
evaluation <strong>of</strong> <strong>aggregation</strong> on surfaces has been
limited to date.
Acknowledgements
We gratefully acknowledge support from: The
Camille and Henry Dreyfus Foundation New
Faculty Start-Up), Research Corporation
Cottrell College Science Award), the American
Association <strong>of</strong> University Women American
Fellowship), HHMI Undergraduate Biological
Sciences Education Progam and NSF-REU.
Finally, SMO would like to thank Pr<strong>of</strong>essor Geri
Richmond and the University <strong>of</strong> Oregon for
providing an ideal environment in which to
complete this work.
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