Can Surface Plasmon Fields Provide a New Way to Photosensitize

Subscriber access provided by RYERSON UNIV

Perspective

Can Surface Plasmon Fields Provide a New Way to Photosensitize Organic Photoreactions? From designer nanoparticles to custom applications. Juan C. Scaiano, and Kevin G. Stamplecoskie J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/jz400002a • Publication Date (Web): 18 Mar 2013 Downloaded from http://pubs.acs.org on March 20, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry Letters

Perspective

Can Surface Plasmon Fields Provide a New Way to Photosensitize Organic Photoreactions? From Designer Nanoparticles to Custom Applications Juan C. Scaiano* and Kevin Stamplecoskie Department of Chemistry and Centre for Catalysis Research and Innovation, University of Ottawa, 10 Marie Curie, Ottawa, Ontario K1N 6N5, Canada [email protected] RECEIVED DATE (to be automatically inserted after your manuscript is accepted)

ABSTRACT: In this perspective we explore the opportunities that plasmon excitation may offer for the practitioners in organic chemistry. Beyond the interesting physical properties and lively colors of colloidal solutions of noble metal nanostructures, excitation of plasmon transitions can trigger a variety of processes, from the simple heat delivery with pinpoint precision, to the enhanced generation of excited states in the immediate vicinity of the nanoparticle, to electron and hole transfer processes that can readily participate in photoredox processes. In understanding how particles are produced, what properties they have, and the diversity of nanostructures and environments in which they can be produced, we aim at providing the small steps towards a paradigm that will allow organic chemists to take advantage of the opportunities that await in the area of plasmon-assisted processes.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

Surface plasmon excitation offers fascinating opportunities in organic and biological chemistry.1 The idea that molecules can be excited electronically in spectral regions where they do not absorb, or that thermal energy can be delivered and targeted to specific molecules or biosystems opens many opportunities. This Perspective explores the intriguing opportunities that arise through plasmon-based excitation of noble metal nanoparticles and the mechanisms by which these interactions can occur; some emphasis is placed on gold nanoparticles (AuNP). Particles to be utilized in plasmon excitation studies can originate from any of the synthetic methodologies available; in the case of AuNP reduction of AuCl4– with either citrate or borohydride are popular methods.2 In our laboratory we have emphasized photochemical techniques that usually rely on the generation of reducing radicals,3-5 in particular, ketyl radicals or α-aminoalkyl radicals, both excellent electron donors. In the first part of this article we outline the benefits of some of the photochemical methodologies we have developed, then describe the characteristics of plasmon excitation from the perspective of an organic chemist who sees it as an intriguing tool, and finally provide an overview of research from our laboratory and elsewhere, where plasmon excitation has been used to advantage. TABLE OF CONTENTS GRAPHIC

keywords: plasmon, metal nanoparticles, colloid, field enhancement, photochemistry, photothermal

Photochemical Synthesis of Metal nanoparticles A few years ago we were surprised by the wide range of exposure conditions that had been reported for the photochemical synthesis of noble metal nanoparticles; given that these reactions are usually performed in millimolar solutions, some irradiations appeared too long for the conversions achieved. In 2 ACS Paragon Plus Environment

Page 3 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


other words, we were curious about the apparently low quantum yields of nanoparticle synthesis reactions. Two free radical initiators have proven particularly useful, and their photochemistry, which involves Norrish Type I cleavage,6,7 is illustrated in Scheme 1.

Scheme 1. Photochemical Norrish Type I cleavage of Irgacure 2959 (I-2959) and Irgacure 907 (I-907).

The decomposition of I-2959 produces the 2-hydrox-2-propyl radical, i.e., the same ketyl radical as from acetone, while I-907 produces an α-aminoalkyl radical; the latter is a better reducing agent, but in some respects less convenient than I-2959, since the amine moiety in I-907 can in itself behave as a slow reducing agent.

In general, I-2959 is utilized in stoichiometric amounts based on the number of

electrons required to reduce a metal ion, e.g. 1:1 for Ag+ and 3:1 for Au3+. For simplicity the reaction is illustrated for Ag+ in Scheme 2. Since these reactions are usually performed in millimolar solutions and a stoichiometric amount of protons is generated, it is common for the final pH to be around 3. In most cases the pH can be readjusted post-synthesis,


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 32

Scheme 2. Photocleavage of I-2959, followed by reduction of Ag+ and formation of silver nanoparticles (AgNP). Oxidation of the benzoyl radical is probably mediated by the peracid, but it is eventually the carboxylic acid that contributes to particle stabilization.

We were initially surprised, particularly in the case of AuNP, by the fact that I-2959 appears to make more stable particles if air is not excluded from the reaction. We now believe that the carboxylic acid, derived from the benzoyl-type radical (see Scheme 1) contributes to the nanoparticle stability.8 The formation of aqueous nanoparticles under UVA irradiation takes place in a few minutes, under conditions of illumination typically involving doses in the 40-80 W/m2 range,4 see Figure 1.

Figure 1. Absorbance vs. time due to aqueous gold nanoparticles under air prepared with 0.33 mM HAuCl4 and 1.0 mM I-2959 exposed to 40 W/m2 UVA at 1 min intervals. The spectra were recorded 24 h after exposure to allow for particle ripening. Note that the reaction is complete in 6-8 minutes. Based on Figure 1 from an earlier report.4

The factors that control the efficiency of photochemical nanoparticle formation became clear from these studies. Many transition metal ions, including Ag+, Au+, Au3+ and Cu2+ are excellent quenchers of excited states, frequently with quenching rate constants in excess of 109 M–1s–1.3 As a result, if the photochemical precursor for reducing free radicals (such as ketyl radicals) has a long triplet lifetime (for example, microseconds), the metal ions in millimolar concentrations can quench the excited state ACS Paragon Plus Environment

4

Page 5 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


precursor before this excited molecule can generate the required free radicals. This constitutes a major reaction inefficiency that results in long irradiation times. In contrast, the triplet state of I-2959 has a lifetime of only 11 ns and it cleaves (see Scheme 1) with a quantum yield of 0.29.6 Under normal experimental conditions the vast majority of the I-2959 triplets escape metal ion quenching and as a consequence irradiation times can be quite short. For example, the typical concentration of AuCl4– in a synthesis of AuNP is 0.3 mM;4 if we assume the rate constant for quenching of triplet I-2959 to be the same as for xanthone (probably a good assumption, since it is a diffusion controlled process),3 then, the efficiency of triplet quenching is given by:

φ quenching =

φ quenching =

k q [quencher]

τ T –1 + k q [quencher]

=

k q [AuCl4 – ]

τ T –1 + k q [AuCl4 – ]

1.03 × 1010 M –1s–1 × 0.0003M = 0.033 (11 × 10 –9 s) –1 +1.03 × 1010 M –1s–1 × 0.0003M

(1)

(2)

That is, under the conditions above, about 97% of the triplets escape quenching. In a detailed study of the photochemical strategies to make AuNP it became clear that the approach for efficient nanoparticle synthesis requires protection of the excited state precursor from quenching, something that can be based on time or space segregation.3 The strategy above is based on time (that is, short triplet lifetime), while spatial separation can be achieved by using supramolecular structures, such as micelles.9 Recent results from our laboratory suggest that the triplet state of some ketones, including benzophenone, can transfer an electron to noble metal ions (just as they do to methyl viologen),10 and lead to nanoparticle formation. The quantum yields are lower than in the case of I-2959. Our group has been interested in producing unprotected (or ideally “naked”) nanoparticles; while most nanoparticles will normally find applications that involve either supports or surface coverage, the best way to make these materials is starting from a clean surface to which the desired coverage is added. In other words, a particle that is born clean is better than one that has been cleaned. We have examined a


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 32

number of reducing agents that yield only volatile products.11 Among these the most interesting one involves the use of hydrogen peroxide as a reducing agent, as illustrated in Scheme 3.

Scheme 3: Opportunistic use of hydrogen peroxide as a reducing agent in the synthesis of AuNP11 taking advantage of the chlorine atom formed by photolysis of AuCl4– .

An ideal paradigm for the synthesis of nanoparticles would include control of size, shape, aggregation, surface coverage and surrounding environment (e.g., solvent).

Interestingly, while not perfect,

photochemistry allows considerable control of all these properties. For example, particle size can be controlled through light intensity (stronger illumination leads to smaller particles),4 by using small particles as seeds of further photochemical or thermal growth,12 by arresting particle growth during synthesis with benign capping agents, such as citrate, or, as we demonstrated for AgNP, by LED irradiations combined with a judicious choice of the irradiation wavelength.13 Interestingly, the reverse, particle size reduction can be achieved by laser ablation of larger particles,14 while reshaping (maintaining constant the number of atoms) can be achieved by femtosecond excitation below a given threshold.15 The synthesis of small monodisperse particles is important in many fields, including catalysis16,17 and the control of melting points; the latter are remarkably low for nanostructures.18 In the case of AgNP, shape control can be achieved by illumination (LED) at different wavelengths; this type of shape-control is a growth process, and as a result shape and size are not independent parameters.19 Many methodologies have been reported to obtain a wide range of nanoparticle shapes and it is not justified to repeat them here.20 Suffice to say that a combination of thermal and photochemical techniques can be used to obtain particles of a pre-selected shape. It is not intended here ACS Paragon Plus Environment

6

Page 7 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to have an exhaustive review of shape control of Ag and Au nanoparticles. For further information one is directed to several other reviews and works.19-21,22,23 If there is one clear advantage of the use of photochemical techniques to produce nanoparticles, it is an easy control of the molecules responsible for surface coverage, and the exchange of them readily if needed. Photochemical methods, such as AuNP synthesis from NaAuCl4 and I-2959 yield particles with excellent stability, yet only with mild, easily removable surface coverage. In this example the surface contains chloride and the carboxylic acid of Scheme 2. The particles show a negative zeta potential, and the absence of covalently bound stabilizers results in easy replacement by the desired coverage. Finally, control of the surface coverage is intimately linked to the ability to change the environment of the nanoparticles, such as the solvent. In fact, it is not the metal, but rather its surface stabilizers, that will determine the nanoparticle affinity for a given media. Thus, photochemistry is a useful tool to produce nanoparticles of controlled size and shape under exceptionally mild conditions, and control its coverage in order to tune its compatibility with different media. Photochemistry has also proven useful as a tool to make bimetallic nanoparticles, of either the coreshell type, or actual alloys.12,24,25 While the synthesis of bimetallic particles is not covered here, it is worth noting that this is an important tool to control the spectral characteristics of the surface plasmon band, a central topic in the next section.

Energy delivery through plasmon excitation Figure 2 shows that spectra of the surface plasmon band (SPB) for spherical nanoparticles of Au, Ag and Cu, with maxima at ~530, ~400 and ~580 nm.26 These SPB transitions are not ‘fixed’ in that their exact position and bandwidth are sensitive to size and environment, even for spherical nanoparticles. Further, nanoparticle aggregation can lead to major changes in the spectra due to coupling of individual particle plasmons.23


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 32

Figure 2: Representative SPB absorbance spectra for spherical Ag, Au and Cu nanoparticle colloids in water.

Surface plasmons are frequently described as the collective oscillations of conduction band electrons following variations of the electric field vector of the incident beam.15,23,27 Simplistic, yet correct, this description fails to give most chemists any idea of what these plasmons could be used for in their field of chemistry. Figure 3 shows a schematic representation of a wave and a nanoparticle; it has been scaled so that if the excitation wavelength were 530 nm (typical AuNP plasmon) the particle size would be 20 nm. Given that the AuNP is small compared with the wavelength of the light, one can assume that all electrons in the AuNP see the same field at any given time; this is described as the quasi-static approximation.28 In fact, if the particle is viewed at 45° (see Figure 3), the field variations between the center and the AuNP edge would be ±13%. The polarized electrons undergo a restoring force that opposes that of the external field; for this wavelength the frequency is 5.66 x 1014 s–1, that is the field changes sign every 0.9 fs. In contrast with molecular structures where the energy content of the molecule can usually be associated to a specific excited electron, in the case of plasmon transitions the energy is ‘shared’ by many electrons as they participate in this collective process.


8

Page 9 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Schematic description of nanoparticles under the effect of an electromagnetic wave, scaled so that for a wavelength of 530 nm the particle size would be 20 nm.

This perspective aims at providing some ideas of how these SPB transitions in nanostructures could be used for the following objectives: 1. Lead to the formation of excited states, both singlets and triplets through processes sometimes described as transmitter-receiver antenna interactions.29,30 2. Cause intense heating of molecules or moieties in close proximity to the surface.31,32 3. Electron and hole transfer reactions between the metal nanoparticle and molecules at or near the surface by enhancing and interacting with excitons. Examples of all three types of processes are rapidly emerging, particularly of the first two, while electron exchange processes, somewhat less characterized, are probably important in many examples of plasmon mediated photocatalysis. Figure 4 illustrates the various processes that can result from SPB excitation. Given the short duration of plasmon excited states (reportedly in the femtosecond time domain), it seems safe to assume that molecular diffusion will be negligible in this time scale. Thus, for molecules to be influenced by SPB excitation, they must reside within the region of SPB influence at the time of photon absorption. We will see below that this generally means within 20 nm from the metal surface. ACS Paragon Plus Environment

9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 32

Figure 4. Plasmon excitation leads to a range of effects on molecules adsorbed, bound, or in proximity to a nanoparticle, as well as changes in the nanoparticle itself. In the presence of a suitable receiver, antenna effects (A) can result in excited state processes. Plasmon relaxation can lead to thermal effects (T) that can themselves induce supramolecular changes in guest molecules (Ts), chemical change (Tc) sometimes referred as photocatalysis, or changes in the nanoparticle itself (Tn) of either a physical or chemical nature (e.g., oxidation). Under plasmon excitation the nanoparticle can act as an electron donor (E) or as an electron acceptor (H) for molecules experiencing enhanced excitation as well (A).

In a remarkable experiment, Novotny and coworkers29 were able to monitor the emission of a single dye molecule fixed on a surface as an 80 nm AuNP, fixed to a tip approached the molecule. This experiment demonstrated that the dye molecule experienced three regimes as the AuNP approached. These are shown in Figure 5, illustrating the key observation from Novotny’s work.


10

Page 11 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Representation of the interaction of a dye with the AuNP surface. Quantitative data adapted from a literature report showing the observed luminescence enhancement (normalized fluorescence rate) for different dye-AuNP distances (z).29 Three regions are identified at the top corresponding to predominant surface quenching (1), emission enhancement (2) and negligible dye-AuNP interaction (3). Reproduced with permission from ref 29.

At very long distances (>20 nm) the AuNP has virtually no influence on dye excited state behavior. As the distance between dye and surface decreases, the behavior of the dye is influenced by the increasing strength of the plasmon field, an effect that has been described as a transmitter-receiver pair, with the AuNP as the transmitter and the dye as the receiver;29,30,33 Experimental observations include enhanced Raman signals (SERS), enhanced fluorescence, enhanced triplet formation30 and enhanced absorption;34 this last one is very important, since quite frequently the other observables (e.g., enhanced fluorescence) incorporate changes in substrate absorbance induced by the plasmon field. Fluorescence


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 32

enhancements of a factor of 10 have been reported for dyes with near unity fluorescence quantum yield.35 In the final approach to the surface, at distances 10,000 M-1cm-1; for example for coumarin-34340 the value is 44,300 M-1cm-1 and the extinction coefficient for a 20 nm AgNP is 4.75 x109 M-1cm-1 (see Figure 8, 20 nm particle has 2.5 x105 atoms).24 This means that the transition dipole moment for a silver nanoparticle is roughly 300 times as strong as that for coumarin-343, and so the electromagnetic field effect on a neighboring molecule is also 300 times stronger for a 20 nm silver nanoparticle. As particle size increases, extinction coefficient of the particle increases drastically as well, so this effect is even higher for larger NP. This is meant only as a conceptual understanding showing how NP can be considered to be a very strong oscillating electromagnetic field to enhance excitation of neighboring molecules. In the vicinity of a nanoparticle, not only the excitation, but also the quantum yield of emission can be increased for a molecule near an excited particle. There is always a competition between the quenching and enhancement effects, but the distance dependence for each is different, so that quenching dominates very close (

Can Surface Plasmon Fields Provide a New Way to Photosensitize

Recommend Documents