Single-molecule imaging of platinum ligand exchange reaction reveals reactivity distribution

Article abstract of DOI:10.1021/ja105517d

Single-molecule fluorescence microscopy provided information about the real-time distribution of chemical reactivity on silicon oxide supports at the solution-surface interface, at a level of detail which would be unavailable from a traditional ensemble technique or from a technique that imaged the static physical properties of the surface. Chemical reactions on the surface were found to be uncorrelated; that is, the chemical reaction of one metal complex did not influence the location of a future chemical reaction of another metal complex.

Full text of DOI:10.1021/ja105517d

Published on Web 08/23/2010
Single-Molecule Imaging of Platinum Ligand Exchange Reaction Reveals
Reactivity Distribution
N. Melody Esfandiari,† Yong Wang,† Jonathan Y. Bass, Trevor P. Cornell, Douglas A. L. Otte,
Ming H. Cheng, John C. Hemminger, Theresa M. McIntire, Vladimir A. Mandelshtam, and
Suzanne A. Blum*
Department of Chemistry, UniVersity of California, IrVine, California 92697
Received June 23, 2010; E-mail: blums@uci.edu
Abstract: Single-molecule fluorescence microscopy provided
information about the real-time distribution of chemical reactivity
on silicon oxide supports at the solution-surface interface, at a
level of detail which would be unavailable from a traditional
ensemble technique or from a technique that imaged the static
physical properties of the surface. Chemical reactions on the
surface were found to be uncorrelated; that is, the chemical
reaction of one metal complex did not influence the location of a
future chemical reaction of another metal complex.
The distribution of reactivity traditionally complicates the analysis
of silicon oxide supported metal complexes,1-3 which are used in
numerous catalytic processes, including industrial processes.1,4-6
Modification of the silicon oxide surface with a silyloxy reagent is
a common strategy for tethering the metal complex to the
surface.4,5,7,8 Irregular surface features on glass and silica surfaces
modified with silyloxy reagents have been studied by AFM;9,10
however, a disconnect exists between this information regarding
static surface physical features on the multimicrometer scale and
the chemical reactivity of that surface in solution. Single-molecule
fluorescence microscopy techniques are powerful methods to detect
reactivity distributions in biophysical systems11-22 but have not
yet received widespread adaptation to chemical transformations.23-29
Herein we demonstrate the ability of single-molecule fluorescence
microscopy to bridge the information gap between imaging of the
Figure 1. (a) Chemical reactions on patterned surface, 80 × 73 µm2,
surface’s static physical features and its dynamic chemical reactivity
by revealing real-time information about the spatial distribution of
chemical reactivity of a triethoxysilane-modified surface at the
solution/surface interface. This level of detail would not be available
by traditional analytical methods, including ensemble methods that
average the collective properties of billions of molecules.
The general approach used in our experiments is shown
schematically in eq 1. We examined the binding affinity of
BODIPY-tagged (dien)platinum complex 1 (dien ) diethylenetri-
amine) to glass microscope coverslips modified with N,N′-
[3-(triethoxysilyl)propyl]thiourea.9,10 We anticipated that the reac-
tion of 1 with surface thiourea groups would rapidly immobilize
the complex through platinum-sulfur covalent bond formation to
form 2, in analogy to the well-established ligand exchange chemistry
of 1 in solution (eq 1).30
showing heterogeneous distribution of reactivity after 1.2 s. Each white
spot that appears indicates one Pt-S chemical reaction that occurred during
the imaging; example individual platinum complex circled in red. (b)
Composite image of 0-7.2 s. (c) AFM image of patterned surface, 80 ×
80 µm2 (thiourea, light gray; unfunctionalized glass, dark gray).
rapidly.32 The individual platinum complexes on the surface
displayed quantized binding and photobleaching events, which are
established characterization fingerprints of single molecules (ex-
amples in Figures S5-S7).33 Thus, the appearance of one fluores-
cence signal characterized the surface chemical reaction of one
complex (Figure 1a).
We investigated the ability of the single-molecule technique to
correlate surface physical inhomogeneity with chemical reactivity.
A physically heterogeneous surface was created by patterning
coverslips with alternating 25 µm stripes of thiourea and unfunc-
tionalized glass, using a photopatterning process. The patterned
coverslips were then employed as substrates for the chemical
reaction with 1. Stripes that contained the thiourea functional groups
were found to recruit fluorophore-tagged platinum complexes
significantly faster than stripes that were not functionalized (Figure
1a, 80 × 73 µm2 field of view). In Figure 1a and b, each individual
Weusedtotalinternalreflectionfluorescence(TIRF)microscopy31,32
to image individual surface chemical reactions of 1 because only
platinum complexes that were bound to the surface were detected,
and molecules of 1 that remained in solution were not detected
because they were not excited and/or because they were diffusing
† These authors contributed equally.
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J. AM. CHEM. SOC. 2010, 132, 15167–15169 15167

C O M M U N I C A T I O N S
white spot is one covalently bound platinum complex that under-
went a chemical reaction during the imaging.
The spatial reactivity distribution across the patterned surface
was quantified by measuring the distance between each individual
chemical reaction; this measurement provided a level of detail that
is not available through traditional ensemble experiments. Specif-
ically, we estimated the radial pair correlation function F(r), i.e.,
the probability distribution of finding two binding events separated
from each other by distance r under the reaction conditions (Figure
3a, black). When compared to a probability distribution F(r) for a
generated set of random points uniformly distributed within the
same domain (red curve), the experimental data display a significant
decrease in the number of reactions that occurred at intermediate
distances, consistent with the reactivity anticipated for a striped
pattern.36,37
We were first concerned with the fact that we were imaging Pt-S
covalent chemical reactivity and not surface physisorption.34
A
series of control experiments lacking any sulfur or platinum
functionality confirmed the specificity of the surface chemical
reaction. For example, physisorption of the BODIPY tag accounted
for less than 2% of all binding events, as determined by comparing
the binding of control compound 3 to the thiourea surface to a
separate sample of 1 to the thiourea surface. The attachment of the
platinum complex to the surface thiourea groups was confirmed to
be covalent in nature via XPS characterization of the platinum-sulfur
covalent bond on the surface (Figure 2). Specifically, two doublets
provided the best fit for the XPS data of both 2 and 4, and in both
complexes these peak maxima were at 71.0 and 72.3 eV. The
similarity by XPS between potential thiourea complex 2 and
authentic thiourea complex 4 indicated that the binding of (dien)-
platinum complexes to the thiourea surface was predominantly via
covalent bond formation between the platinum and thiourea sulfur.
The XPS experiments confirming covalent bond formation are
further detailed in the Supporting Information.
Figure 3. (a) Observed radial pair correlation function of 1 on striped
surface, black, and generated uniform distribution with error bars, red.
Difference shows degree of nonuniformity. (b) Observed radial pair
correlation function of 1 within a subset of the thiourea surface, black, and
generated uniform noncorrelated distribution, red, with error bars. Similarity
at short distances established that binding events are noncorrelated.
The ability to localize individual events also permitted the
probing of whether or not the chemical reaction of one platinum
complex influenced the location of a future chemical reaction (e.g.,
if the chemical reactions were correlated). In order to probe this
correlation, the reactions within a 32 µm2 subset of a thiourea stripe
were analyzed. At short distances, where correlation would be most
likely, our data closely match a generated set of noncorrelated data
(Figure 3b).38,39 This similarity established that chemical reactions
on the thiourea surface were noncorrelated; i.e., the chemical
reaction of one platinum complex did not influence the location of
a future chemical event.
In conclusion, the ability to localize single chemical events by
fluorescence microscopy bridges the information gap between the
static physical properties of the silyloxy-functionalized surfaces
detectable by AFM and the distribution of real-time chemical
reactivity at the solution-surface interface on the multimicrometer
scale. The measurement of the distances between each individual
chemical reaction provided a level of detail of the reactivity
distribution that is unavailable through an ensemble technique.
Figure 2. Comparison of XPS raw data and fitting Pt binding energies
shows similarity between surface-bound Pt and authentic Pt-thiourea
complex. Top: 1 on glass surface. Middle: 4, authentic Pt-thiourea covalent
bond. Bottom: Pt bound to thiourea-functionalized surface.
AFM imaging of the patterned surface showed the raised stripes
of thiourea (Figure 1c; light gray) and lower stripes of unfunction-
alized glass (dark gray). Additional 2 µm surface features were
detected at the interface of the thiourea and unfunctionalized glass
regions (white). These features likely corresponded to residual
photoresist at the interface of the two regions.35 Although these
2-µm regions showed different static physical features by AFM,
they shared similar chemical reactivity properties to the thiourea
surface, thereby supporting that these regions were thiourea-coated.
Specifically, the reactive regions were 29 µm wide as measured
by single-molecule fluorescence microscopy, which is the sum of
the widths of the white and light gray surface features visible by
AFM.
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C O M M U N I C A T I O N S
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Acknowledgment. We thank the U.S. Department of Energy-
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Supporting Information Available: Experimental procedures and
compound characterization. This material is available free of charge
via the Internet at http://pubs.acs.org.
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