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20.3 Molecular Imprinting with Diverse Nanomaterials 669

Figure 20.10 Schematic illustration of noncovalent imprinting

of TNT at the surface of acrylate - based nanowires. Adapted

from Ref. [87].

mixture containing the superparamagnetic MnFe

nanocrystallites. Unfortu-

nately, the behavior or potential applications of the magnetic characteristics of

the nanowires were not discussed to any great extent.

Xie et al . described the preparation of TNT surface - imprinted nanowires (Figure

20.10 ), where alumina membranes with pore diameters of 70 nm were prepared

by electrochemical anodization [87] . In these studies, the authors also reported the

ﬁ rst imprinted nanotubes, which were prepared in similar fashion to the nanowires,

the main differences being a reduction in the monomer concentration and an

increase in the quantity of initiator used. The binding capacities of the nanotubes

and nanowires were almost 2.5 - fold and threefold that of normal imprinted parti-

cles with 2 – 3 μ m diameter, respectively. This increase in binding capacity was

attributed to the high ratio of surface - imprinted sites, the large surface - to - volume

ratios, and the complete removal of TNT templates.

Ye and coworkers also described the incorporation of imprinted nanoparticles

into electrospun nanoﬁ bers (as discussed previously in Section 20.2.2.1 ) [66] . The

same group, however, also achieved the generation of artiﬁ cial molecular recogni-

tion sites in the nanoﬁ ber itself by molecular imprinting [88] . The electrospun

nanoﬁ bers were prepared from a solution mixture of PET and polyallylamine

(which acts as a functional polymer) in the presence of a template molecule,

2,4 - dichlorophenoxyacetic acid. Following template removal, the recognition prop-

erties of the imprinted nanoﬁ bers were identiﬁ ed using a radioligand - binding

assay. The imprinted nanoﬁ bers displayed favorable binding characteristics in

aqueous solution, where analyte binding to the imprinted nanoﬁ bers was ﬁ vefold

that of the reference ﬁ bers. The authors predicted that this type of imprinted

nanoﬁ ber would become highly applicable in the future for afﬁ nity - related

separations.

20.3.2

Quantum Dots

Lin and coworkers are, to date, the only group to report the synthesis of

molecularly imprinted polymers in conjunction with ﬂ uorescent semiconductor

nanoparticles also known as quantum dot s ( QD s) [89, 90] . The ﬁ rst step of this

670 20 Molecular Imprinting with Nanomaterials

approach was the preparation of the CdSe/ZnS semiconductor nanoparticles. In

order to ensure that the nanoparticles were incorporated covalently into the

polymer, the surfaces of the QDs were functionalized with 4 - vinylpyridine. The

functionalized nanoparticles were polymerized with MAA and EGDMA in

the presence of uracil, following spin coating of the initiated solution, to form a

molecularly imprinted thin ﬁ lm [89] . In a subsequent study, the QDs were incor-

porated into bulk polymers, which were imprinted with a number of template

molecules including caffeine, uric acid, L - cysteine, and estriol [90] . The authors

showed that binding to recognition sites in the vicinity of the ﬂ uorescent nano-

particles caused quenching of the photoluminescence emission, presumably due

to the ﬂ uorescence resonance energy transfer between the QDs and the template

molecules. The imprinted polymers demonstrated good selectivity towards the

template when compared to structurally related compounds and, according to the

authors, the control polymers exhibited no response to the analyte.

20.3.3

Fullerene

The ﬁ rst “ homogeneous nanoscale imprinting system, ” wherein a saccharide was

used as template and covalently bound by two boronic acid groups onto the surface

of [60] fullerene, was described by Shinkai and coworkers [91] . This strategy was

used in order to produce one single binding site per molecule and to avoid the

creation of numerous binding sites with varying recognition properties (as illus-

trated in Figure 20.11 ) [92] . Initially, a template monomer complex, based on

d , l - threitol, was synthesized which could undergo a regioselective and chiroselec-

tive double [4 + 2] cycloaddition with [60] fullerene. Subsequent cleavage of the

template resulted in two optically active cis - 3 bis adducts with opposite chirality in

a ratio of 72.28. Competitive rebinding studies between d , l - threitol and the

imprinted fullerene demonstrated that L - threitol - imprinted (

A ) - 4 and D - threitol -

Figure 20.11 Structural representation of a monomolecular

covalent imprinting strategy with fullerene for the recognition

of sugars. Adapted from Ref. [91].

20.3 Molecular Imprinting with Diverse Nanomaterials 671

Figure 20.12 Schematic diagram of a monomolecular covalent

imprinting strategy with dendrimers for the recognition of

porphyrins. Adapted from Ref. [96].

imprinted (

C ) - 4 rebound the original templates in diastereoselective fashion.

Although, the selectivity in this system was not extraordinary, it should be empha-

sized that this was the ﬁ rst instance in which molecular imprinting was applied

to a system with a soluble nanosized matrix in homogeneous solution.

20.3.4

Dendrimers

Zimmerman et al . more recently used a different strategy, wherein a single por-

phyrin template was dynamically imprinted into a single macromolecule (den-

drimer), in order to prepare molecularly homogeneous hosts each with a single

recognition site [93] . For this, the authors used the covalent approach, where the

porphyrin acted as the core and was covalently bound through ester linkages to

eight third - generation dendrons to form the dendrimer. The porphyrin was cleaved

by hydrolysis, following the crosslinking of homoallyl end groups at the exterior

of the dendrimer by Grubbs ring - closing metathesis (Figure 20.12 ). Complexation

672 20 Molecular Imprinting with Nanomaterials

studies (where rebinding occurred noncovalently) demonstrated that > 95% of the

imprints were effective, and their binding properties homogeneous [96] . The

imprinted dendrimer was also shown to be selective, and to bind porphyrins with

at least four hydrogen bond donor/acceptor sites, in toluene as solvent. The

imprinted cavity was also size - selective, and although the template itself was too

large to bind, porphyrins of smaller physical size and appropriate functionality

were able to complex strongly.

In a subsequent study, an imprinted dendrimer was prepared around a porphy-

rin with four reactive alcohol groups, which was covalently bound to four fourth -

generation dendrons [94] . More recently, Zimmerman ’ s group reported the

preparation of an imprinted dendrimer with a less - symmetrical template mole-

cule, with a view to producing an amine - selective sensor [95] . Here, very high

afﬁ nity binding was perceived, from a three - point interaction, which included two

hydrogen bonds and one covalent linkage to a reporter group that transduced

binding by color change. This monomolecular imprinting approach resulted in

properties that were difﬁ cult to achieve by conventional polymerization methods,

such as high - efﬁ ciency imprinting, quantitative removal of the template, solubility

of the imprinted material in common organic solvents, and possibility of separa-

tion of imperfectly assembled binding sites. However, the limitations of this

approach were the need for a multistep synthesis of the dendrimers, the high

dilution conditions required for the ring - closing metathesis reaction, and the

incompatibility of many templates with this imprinting approach.

20.4

Conclusions and Future Prospects

Nanomaterials represent one of the most important targets of the developing ﬁ eld

of nanotechnology. The advanced scientiﬁ c knowledge acquired for the synthesis

and physical – chemical characteristics of these materials has led to a considerable

increase in the number of their applications and commercial exploitations. The

application of imprinting technologies to nanomaterials has provided a new

insight, offering the possibility of creating tailored cavities with speciﬁ c recogni-

tion properties.

Over the past decade, considerable effort has been devoted to developing novel

synthetic strategies, and this has led to imprinted materials in a variety of formats.

Attention has been mainly focused on the development of approaches to produc-

ing imprinted nanoparticles, and to providing reproducibility and efﬁ ciency among

the polymerization methodologies used. Although, signiﬁ cant advances have been

made in this area, the widespread commercialization of these materials has yet to

be realized. Yet, ongoing developments in the ﬁ eld of nanomaterials have led to

a range of promising polymeric formats, including soluble crosslinked imprinted

dendrimers, microgels and nanogels, which more closely mimic the physical

characteristics of enzymes and antibodies. Additional formats, such as nanoﬁ bers,

nanowires and nanotubes, have arisen and have been used successfully in

References 673

conjunction with imprinting technologies. These materials exploit the advantages

of high surface - to - volume ratios and often also the surface - imprinting techniques

that allow an easy access of target molecules to recognition sites. The results

obtained to date in this area have been the outcome of an increasing interest in

molecular imprinting, by both academic and industrial enterprises, and will surely

lead to their signiﬁ cant application in the near future.

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Near - Field R aman Imaging of Nanostructures and Devices

Ze Xiang Shen , Johnson Kasim , and Ting Yu

677

Advanced Nanomaterials. Edited by Kurt E. Geckeler and Hiroyuki Nishide

ISBN: 978-3-527-31794-3

21.1

Introduction

Signiﬁ cant advances have been made in recent years in nanoscience and nanote-

chnology. Many applications which involve the use of nanomaterials and devices,

and utilize their unique properties, have now been realized and more are expected

in the near future. Despite these achievements, our understanding of nanomateri-

als and devices remains limited, due partly to the lack of suitable characterization

techniques.

In Raman spectroscopy, molecular vibrations are measured that are determined

by the structure and chemical bonding – as well as the masses – of the constituent

atoms/ions. Raman spectra, as with infrared ( IR ) spectra, are unique in their chem-

ical and structural identiﬁ cations. The technique of Raman spectroscopy is

nondestructive, sample preparation - free, there is no requirement for vacuum

application, and it can be carried out in an aqueous/liquid environment. The

technique can also easily be carried out at a different temperature, pressure, and

electrical and magnetic ﬁ elds. If it were possible to use Raman spectroscopy for

nanocharacterization, it would provide critically important material - speciﬁ c prop-

erties such as composition, chemical bonding, crystal and electronic structures

and strain/stress, as well as supplementary information to other nanotechniques,

such as scanning electron microscopy ( SEM ), atomic force microscopy ( AFM ), and

transmission electron microscopy ( TEM ).

Conventional (far - ﬁ eld) micro - Raman spectroscopy has a spatial resolution of

approximately 0.5 μ m, which is governed by the theoretical diffraction limit,

whereas IR spectrometry has at best a spatial resolution of approximately 10 μ m,

due to the longer wavelengths being utilized. Hence, the application of these

techniques in nanoscience and nanotechnology is severely limited by their poor

spatial resolution. Consequently, extensive efforts have recently been made to

characterize structural information at the nanometer scale, utilizing near - ﬁ eld

scanning optical microscopy ( NSOM ), the different operational modes of which

are shown in Figure 21.1 .

678 21 Near-Field Raman Imaging of Nanostructures and Devices

In this chapter, presented data have been acquired using both far - ﬁ eld and near -

ﬁ eld Raman imaging. The aim is to demonstrate the importance of Raman

imaging in materials and device research.

21.2

Near - Field R aman Imaging Techniques

There are three main approaches to near - ﬁ eld scanning Raman microscopy

( NSRM ). The ﬁ rst approach is based on the principle of NSOM [1 – 3] , which is

also known as “ aperture near - ﬁ eld Raman . ” It is embodied through an aperture

NSOM, whereby an optical ﬁ ber tip with a small aperture (50 – 200 nm) is used to

deliver laser light, while the ﬁ ber tip is held at a close distance (some tens of

nanometers) above the sample surface. The Raman signal is collected in far - ﬁ eld

through either a microscope objective or a lens [4 – 6] . The scattered light (Raman

signal) is coupled to a Raman spectrometer, where the Raman spectra are recorded

at each point on the sample by scanning the ﬁ ber tip across the sample surface.

An NSRM image is constructed using these spectra.

The laser emerging from such a ﬁ ber tip is extremely weak (typically 100 nW)

due to the low optical throughput of the ﬁ ber tip. Hence, aperture near - ﬁ eld

Raman has been plagued by the poor signal - to - noise ratio ( SNR ) of the near - ﬁ eld

Figure 21.1 Different operational modes of near - ﬁ eld

scanning optical microscopy. (a) Illumination mode;

(b) Collection mode; (c) Illumination and collection mode.