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Supramolecular Chemistry Meets Hybrid (Nano)Materials: A Brief Look Ahead

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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



is probably the most serious barrier to the development of reliable continuous monitoring devices. Various protective coatings, such as Nafion or agarose gel, have been

proposed for excluding surface-active macromolecules and minimizing surfacefouling effects. Wang’s group has demonstrated an alternative approach to reduction

of electrode fouling through the use of adaptive nanowires.47 These consist of

alkanethiol-coated gold nanowires, containing a short nickel (magnetic) segment.

The nanowire-based adaptive protective system enables the user to control exposure

of the surface during measurement mode (active) and protection of the surface in

the protective mode (passive) between repetitive measurements (Fig. 23.9). This is

accomplished by switching magnetically the surface orientation of the nanowires,

between vertical and horizontal positions. This leads to opening and closing of the

surface, respectively, to allow the measurement and protect the transducer between

repetitive runs.

The optical images of Figure 23.9 shed useful insights into the protective action of

the bisegment adaptive nanowires. The top view of the vertical nanowires (left) indicates that the island-like bundle structure of the vertically oriented nanowires exposes a

major portion of the surface of the glassy-carbon disk (dark region). This vertical

(active) state thus allows facile deposition and stripping of metals on the electrode

surface and hence convenient electrochemical measurements. In contrast, the surface

is fully covered by these nanowire bundles when the adaptive nanowires are

reoriented in the horizontal position (right). Such a passive state completely blocks

the surfactant access to the surface to offer the necessary protection between repetitive

measurements.



Figure 23.9 Trace analysis of a metal (M) analyte in the presence of surfactants (S) using the vertically

“active” and horizontally “passive” aligned nanowires. Such adaptive operation leads to “opening” and

“closing” of the surface to allow measurement and protection of the transducer between measurements. Also

shown are the optical images (top view) of the glassy-carbon disk electrode covered with the vertically (left)

and horizontally (right) aligned nanowires.47 (Reprinted with permission from R. Laocharoensuk et al.,

J. Am. Chem. Soc. 2007, 129, 7774–7775. Copyright 2007 American Chemical Society.)



23.3 Molecular Recognition/Transduction



667



The ability to switch the operation of electrochemical metal sensors between

active and passive modes on demand offers substantial improvements in their stability

in the presence of common surfactants, as demonstrated in stripping-voltammetric

signals obtained from cadmium in the presence of gelatin and Tween 80. Bare

electrodes display a substantial diminution of the cadmium peak in the presence

of both surfactants. In contrast, the adaptive-nanowire electrode system exhibits a

highly stable response with a negligible change of the peak current over multiple

measurements.



23.3.2



Optical Sensors



Typically an optical sensing platform consists of the molecular recognition and transduction agents immobilized within a membrane or on an active surface, and this is

exposed to the sample. Such exposure of chemically active surfaces to real samples

can lead to changes in the surface and bulk characteristics over time, due to various

interactions with the sample. Active membrane components may leach out into the

sample, binding sites may be blocked or changed in form, or surfaces may become

fouled. Over the past several years, we have investigated the concept of adaptive optical sensing materials based on the following principles:





The sensor surface should be in an inactive or passive state when a measurement is not being conducted.







The surface is converted into an active state under an external stimulus (optical

in this case).

The active surface binds with the target species and generates a signal that

enables the analytical measurement to be made.

After the measurement is completed, the target species is expelled by an external stimulus (optical) and the surface returns to its inactive form.









In this way, it may be possible to maintain sensing surfaces in an inactive form

that would remain relatively unchanged over time, potentially extending the sensor’s

useful lifetime by minimizing poisoning effects.

Within our research group we have explored the opportunities afforded by surface-based photoswitchable chemical sensors.48,49 We focus on spiropyrans and

related systems like spiroxazines, as they are a well-studied system that can be photonically switched between two states.50–54 Spiropyran (SP) undergoes a heterocyclic

ring cleavage at the CZO spiro bond that results in the formation of a planar, zwitterionic, and highly conjugated chromophore that absorbs strongly in the visible region,

this being the merocyanine (MC) isomer, which exhibits metal ion-binding behavior

resulting in a shift in the absorbance spectrum, and a corresponding color change, see

Figure 23.10.55,56 For certain divalent metal ions, solution-phase studies have shown

that a 2 : 1 complex of the form MC2-metal ion exists. Consequently, it was thought

that covalent immobilization of the spiropyran molecules to a polymer backbone

might inhibit or completely eliminate the ability of the MC-metal ion complex to

form. The immobilization strategy therefore needed to allow enough flexibility for



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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



Figure 23.10 Representation of photocontrolled ion-binding at an SP-modified surface. (a) Colorless

SP-immobilized surface. (b) On illumination with UV light, the surface becomes active and bright purple

due to the photoisomerization of SP to MC. On illumination of this surface with visible light MC is switched

back to SP. (c) Exposure of activated surface to an aqueous solution of divalent metal ions leads to formation

of the complex MC-Mỵ and further color change of the surface. Irradiation of this surface with green light

leads to transformation of MC-Mỵ to SP. The cycle is closed and the surface is returned to the passive,

colorless state.



the spiropyran molecules not only to photoisomerize between the active and passive

forms, but also to coordinate with the metal ions in the correct stoichiometric ratio.

We therefore used a series of diamino alkyl linkers, ranging in length from two to

eight carbons, as a method to covalently attach a carboxylate version of the spiropyran

to poly(methyl methacrylic acid). The spiropyran-modified polymer substrates were

treated to a series of experiments to determine whether the tether length influenced

the efficiency of photoswitching, ion complexation, and photoinduced dissociation

of the MC-metal ion complex, and regeneration of surface-bound spiropyran for

further measurements. It was expected that formation of the 2 : 1 complex would be

affected by molecular flexibility, and therefore tether length would be important. It

was found that the ability of the SP to photoisomerize dramatically improves with

increasing tether length, as evidenced by the increasing absorbance at 570 nm, the

absorbance maximum for the covalently bound MC form. This was also the case

for metal ion complexation; see Figure 23.11 for UV-vis spectra of MC complexation

with cobalt(II) chloride. It was observed that metal ion complexation only occurred

when there was a separation of at least eight carbon spacer atoms between the SP molecule and the polymer backbone. We also found it was possible to dissociate the

MC-Co2ỵ complex with visible light. This was the first demonstration of the cycling



23.3 Molecular Recognition/Transduction



669



Figure 23.11



UV-vis analysis of photocontrolled ion-binding. (a) Regeneration of spiropyran and

merocyanine Co2ỵ complex in acetonitrile through the following steps: (1) spiropyran, (2) merocyanine,

(3) complex, (4) spiropyran, (5) complex. (b) Cycling of spiropyran to merocyanine and generation of

merocyanine-Cu2ỵ complex on SP-modied surface, 560 nm (dashed line) and 431 nm (full line).



of covalently immobilized spiropyran on a solid support to merocyanine for repeat

cycling through uptake, detection, and release of metal ions.

These results demonstrate that a spiropyran-modified polymer can adapt its functionality through reversible molecular rearrangements triggered by external stimuli

(photons). In principle, this means that the immobilized chemorecognition sites can

be maintained in an inactive or passive form until a measurement is required. At

this point, the surface is illuminated with UV photons, which triggers the molecular

rearrangement to the active form. Furthermore, the polymer is self-indicating, as the

presence of the active form is easily identified via the intense purple color, and therefore some degree of self-diagnostics can be easily incorporated into measurements. In

the active form, binding with metal ions such as Co2ỵ, Cu2ỵ, and Cd2ỵ can occur,55,56

and once again it is self-indicating, as complexation shifts the absorbance of the active

site and the color changes to pink from purple. When the measurement has been completed, illumination with white light expels the guest ion and returns the polymer to the

inactive (colorless) form. The implications of this research are that it may be possible,

using this approach, to maintain a sensing surface in a passive mode that does not

interact significantly with the external environment. When a measurement is required,

the active form can be created and the population of active sites monitored via the

intensity of the purple color. The presence of the target species (e.g., Co2ỵ) can

then be measured by ratioing the absorbance at, for example, 430 nm and 570 nm.

A decrease at 570 nm with an accompanying increase at 430 nm is indicative of the

presence of Co2ỵ. This raises the prospect of having sensing surfaces that do not

change characteristics in a significant manner over time, potentially extending the lifetime of the sensing surface. The self-indicating nature of spiropyran is another powerful feature, which provides a degree of self-diagnostics and internal referencing of

analytical measurements. In addition, covalent attachment to the polymer substrate

prevents leaching of active sites into the sample. Samanta and Locklin57 have designed

photochromic polymer brushes for photoswitchable surface wetting based on our

polymer merocyanine-Co2ỵ complex. They reported a photoinduced enhancement

of contact angle change with increasing linker length of spiropyran from the substrate

surface. Perhaps more importantly, they also observed a significant enhancement of



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Chapter 23 Molecular Schizophrenics: Switchable Materials with Multiple Functions



contact change (approximately 35 degrees) when the spiropyran-modified substrate

was irradiated in the presence of cobalt(II) chloride. To our knowledge, this is the

largest photoinduced change in contact angle that has been observed on a flat surface.

More importantly, this photoinduced surface wetting was shown to be reproducible, up

to five times, with no sign of degradation.57 These results are direct confirmation

through independent techniques of an interpretation of the switching and binding

behavior of these molecular photoswitches.

Furthermore, we show the excellent potential of light-emitting diodes as light

sources and detectors for photoswitching between the two isomeric states of

spiropyran and measurement of ion complexation, see Figure 23.11b.58,59 A simple,

low-cost, low-power experimental setup provides spatial and temporal control of

surface illumination and surface binding. This, coupled with low irradiance, is

shown to generate significant improvement in fatigue resistance of SP-modified polymeric films, and may prove to be an important step towards more sophisticated

materials capable of switching reversibly between active and passive forms, and simultaneously providing a number of transduction modes for gathering information

about the molecular environment in the immediate vicinity of the binding site when

in the active mode.



23.4 CONCLUSIONS

The key to many future disruptive technologies lies in the development of materials that

exhibit stimulus-responsive behavior. The area has advanced rapidly in recent years, as

the science and technology of molecular and nanoscale control and characterization of

the surface and bulk structure of materials continues to develop. The range of materials

that can be switched between dramatically different modes of behavior is expanding

rapidly, and in this chapter, we have only been able to provide a high level introduction

into some of the exciting possibilities that can arise from these developments—

materials whose physical and chemical properties can be controlled using an external

stimulus. These materials have the potential to be incorporated into a wide range of

specialist and consumer products within the next five years that could dramatically

impact on society. Furnishings and clothes that change color, textiles that can sense

and communicate, chemical sensors whose surface binding activity can be turned on

and off, and microfluidic systems with biomimetic components such as soft polymer

pumps and valves that could revolutionize the way analytical measurements are

made. All in all, it seems clear that there are exciting times ahead in sensor science

aligned with adaptive or stimuli-responsive materials!



ACKNOWLEDGMENTS

We would like to acknowledge support from Science Foundation Ireland under

the CLARITY grant (07/CE/I1147) and Light Activated Molecular Switches

(07/RFP/MASF812) awards.



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Chapter



24



Hybrid Nanomaterials

Research: Is It Really

Interdisciplinary?

ISMAEL RAFOLS, MARTIN MEYER, AND JAE-HWAN PARK

24.1 INTRODUCTION



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24.2 THE CURRENT RISE OF INTERDISCIPLINARITY IN CONTEXT



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24.3 THE ASSESSMENT OF INTERDISCIPLINARITY



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24.4 DATA AND METHODS



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24.5 KNOWLEDGE STRUCTURE IN HYBRID NANOMATERIALS

24.5.1 MAIN TOPICS OF RESEARCH CLUSTERS

24.5.2 DISCIPLINARY DIVERSITY

24.5.3 DRIVERS OF KNOWLEDGE INTEGRATION



678

678

680

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24.6 SUMMARY AND CONCLUSIONS



685



ACKNOWLEDGMENTS



686



REFERENCES



687



24.1 INTRODUCTION

One of the central tenets of the current discourse on innovation is that the most important scientific and technological breakthroughs are the result of interdisciplinary

endeavors. This idea has become particularly significant in those areas of science

and technology that are perceived as being a result of technological convergence,

such as nanotechnology or its cognate developments:

Revolutionary advances at the interfaces between previously separate fields of science

and technology are ready to create key NBIC transforming tools (nano-, bio, info-, and



The Supramolecular Chemistry of Organic–Inorganic Hybrid Materials. Edited by Knut Rurack and

Ramo´n Martı´nez-Ma´n˜ez

Copyright # 2010 John Wiley & Sons, Inc.



673



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Chapter 24 Hybrid Nanomaterials Research: Is It Really Interdisciplinary?

cognitive-based technologies), including scientific instruments, analytical methodologies,

and radically new materials systems. The innovative momentum in these interdisciplinary

areas must not be lost but harnessed to accelerate unification of the disciplines. (Roco and

Bainbridge,1 p. 3)



The field of hybrid organic – inorganic nanomaterials explored in this book fits

nicely with this view of research as driven by applications whose development

requires interdisciplinary effort. Along these lines, recent reviews have presented

hybrid organic – inorganic nanomaterials as exploiting interdisciplinary interactions

between supramolecular chemistry, materials science research, and nanotechnology.2

However, interdisciplinarity is a rather nebulous and polysemous concept. A giant collaboration building a high energy collider is surely very different from a project on climate change in a social science institute, or a laboratory working on molecular motors.

Although each of these endeavors may claim to be interdisciplinary, the underlying

cognitive and social processes are markedly dissimilar.

In which sense is hybrid nanomaterials research interdisciplinary? Of the many

existing perspectives on interdisciplinarity, this chapter presents an exploration of

hybrid nanomaterials based on the knowledge sources of the field, using bibliometric

data. Since this data is limited to the contents of the book, it should be stressed that the

analysis and conclusions pertain to hybrid nanomaterials in the sense of this book, that

is, in terms of supramolecular chemistryÃ. The main findings are that hybrid nanomaterials research is very fragmented among different materials-centered clusters

[e.g. carbon nanotubes (CNTs) and quantum dots (QDs)], and applications-centered

topics, with each of these topics drawing on disciplines from chemistry and materials

sciences, and to a lesser extent the biological sciences.

The chapter is organized as follows. We begin with a “cautionary” review of the

current interest in—and rhetoric on—interdisciplinarity in the context of the new discourses on science,3 which stress the need for science to be legitimated by producing

explicit social benefits. Second, we briefly review various approaches to the assessment of interdisciplinarity and present the conceptual framework and methodology

of this investigation. Third, we show and discuss the empirical results and, finally,

we summarize the findings.



24.2 THE CURRENT RISE OF INTERDISCIPLINARITY

IN CONTEXT

Since the early 1990s there has been a boom in the (self-reported) adoption of interdisciplinarity by both scientists and policy makers.4 Occurrences of the term interdisciplinary in scientific papers increased fivefold in 15 years, from 550 per year in 1993

to more than 3100 in 2007.† Policy reports have highlighted the importance of interdisciplinarity for strategic technologies such as nanotechnology.5 There has been a

Ã

Therefore, one should be cautious when extrapolating the conclusions from supramolecular chemistry to

other fields related to hybrid nanomaterials, such as catalysis or separation research.



This search was carried out using ISI’s Science Citation Index database (i.e. not including the Social

Sciences or Humanities) for Articles, Reviews, Notes and Letters only.



24.2 The Current Rise of Interdisciplinarity in Context



675



surge in programs fostering interdisciplinary collaboration and technological convergence, such as NEST (New and Emerging Science and Technology) in the 6th EU

Framework Programme, and large investments have been made into interdisciplinary

research centers, such as Cornell’s Nanobiotechnology Center (NBTC).

While this enshrinement of interdisciplinarity as a positive scientific norm may

today seem “natural,” we should remember that it runs contrary to the beliefs of the

early twentieth century:

Only by strict specialization can the scientific worker become fully conscious, for once and

perhaps never again in his lifetime, that he has achieved something that will endure.

A really definitive and good accomplishment is today always a specialized accomplishment. And whoever lacks the capacity to put on blinders, so to speak, . . . may as well stay

away from science. (Weber,6 p. 135)



Hence, this surge in interdisciplinarity needs to be put into context. Although

some interdisciplinary academic and research programs were developed in the midtwentieth century,7 interdisciplinarity first emerged as a social and policy issue at

the time of student unrest over higher education reforms in the 1960s and 1970s,8

and then regained prominence in the 1990s with the advent of a series of policy studies

claiming that the science and technology system was undergoing major structural

changes.9 Among these studies, the most influential are Gibbons et al.’s contributions,3,10 which depict a transition towards a new research mode (Mode 2) characterized by production of knowledge in the context of application, with more transient

organizational settings, wider societal considerations for the evaluation of its worth,

and which includes transdisciplinary research and heterogeneity of skills among its

key attributes. Other models that have focused on the stronger interaction between

academia and economic or government actors, such as Leydesdorff and Etzkowitz’s

Triple Helix,11 also suggest indirectly a move towards greater interdisciplinarity in

order to address societal or industrial demands.

However, the fact that this literature is prescriptive rather than descriptive suggests

that “the discourse on interdisciplinarity is, in effect, a discourse on innovation in

knowledge production,” (Weingart,8 p. 30) where “interdisciplinarity provides a

means for steering and coordinating strategic investment in research across a range of

partners,” (Lowe and Phillipson,12 p. 167). In other words, science is being pushed

to be more interdisciplinary in order to better fulfill its new social contract, which

entails a more direct interaction with societal actors via technology transfer or public

engagement. In this context, the call for interdisciplinarity may become a convenient

tool used to foster reform in scientific institutions (e.g., to justify the erosion of the

tenure track system13), rather than a requirement of “organic” or “internal” developments

in science (as we might argue was previously the case in areas such as biophysics).

In summary, the current discourse on science implicitly takes a prescriptive stance

and believes that research ought to be more interdisciplinary in order to be more successful at dealing with societal problems and supporting innovation and competitiveness. However, since you can’t get an ought from an is, when an emergent field such as

hybrid nanomaterials is presented as interdisciplinary, one cannot help but wonder: is

it really? Or is it rather that there is an expectation (or desire?) that it become

interdisciplinary?



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Chapter 24 Hybrid Nanomaterials Research: Is It Really Interdisciplinary?



24.3 THE ASSESSMENT OF INTERDISCIPLINARITY

There is an increasing consensus (e.g., as shown in the report on interdisciplinarity of

the U.S. National Academies14) that the key characteristic of interdisciplinarity is

knowledge integration, that is, the combination and/or the fusion of concepts or

theories, tools or techniques, and information or data from various bodies of specialized knowledge. However, there is no agreement in the literature about how to assess

the degree of interdisciplinarity:15 it has been measured in a variety of ways, including

the affiliations of researchers involved in collaborations,16 their educational background,17 the citation flows among disciplines,18 and the co-occurrence of disciplines

in references/citations,19 or in article headings.20

In our view, there are three questions that need to be raised with regard to the

degree and type of interdisciplinarity of a field:‡

1. How diverse and how coherent is its cognitive structure? This question

explores which bodies of knowledge the field is building on, and to what

extent these fields are becoming integrated.

2. What are the drivers of its knowledge integration? This point aims to elucidate

why the emergent field needs to reach out to various knowledge sources.

3. What are the strategies for knowledge acquisition? This examines how laboratories garner knowledge from various disciplinary bases.

The first question looks into interdisciplinarity proper. Since interdisciplinarity is

an epistemic characteristic, it has to be assessed by looking into the contents of

research (e.g., examining the concepts or techniques used, or analyzing their proxies,

such as the structure of publications) rather than its social practices (e.g., collaborations). The second question (why) enquires into the motivations for pursuing

interdisciplinary research and can be investigated by looking into the characteristics

shared among the different bodies of knowledge that are brought together (e.g., instrumentation or application objectives). Schmidt21 proposed that these drivers can be

epistemological (sharing theories and concepts, e.g. in complex systems studies),

methodological (e.g., the atomic force microscope), ontological (i.e. objects-centered,

such as carbon-nanotubes) or problem-oriented (e.g., climate change research).

Finally, the third point (how) looks into the social mechanisms and processes that

constitute the practice of interdisciplinarity (such as recruitment, interaction within

joint facilities, sharing of research material or data, etc.). Here we should like to

emphasize that interdisciplinary research “does not necessarily imply collaboration

between researchers from different disciplines,” (Bordons et al.,15 p. 440) and even

when it does, the term collaboration encompasses a variety of practices.22,23 This is

why in hyped fields such as nanotechnology, where the organizations and educational

degrees are sometimes relabeled without much substantial transformation in order to

cater to shifting funding demands, the number of collaborations among diverse



The questions apply equally to enquiry into interdisciplinarity of organizations, but for the purposes of this

chapter, we will focus on research fields, themes, or topics.



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