A Nanosensor for Explosives Detection Based on 

Molecularly Imprinted Polymers (MIPs) and Surfacedenhanced Raman Scattering (SERS)

by Ellen Holthoff and Dimitra Stratis-Cullum

ARL-TR-5092 March 2010

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Army Research Laboratory

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ARL-TR-5092 March 2010

A Nanosensor for Explosives Detection Based on 

Molecularly Imprinted Polymers (MIPs) and Surfacedenhanced Raman Scattering (SERS)

Ellen Holthoff and Dimitra Stratis-Cullum

Sensors and Electron Devices Directorate, ARL

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A Nanosensor for Explosives Detection Based on Molecularly Imprinted 

Polymers (MIPs) and Surfaced-enhanced Raman Scattering (SERS)

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Ellen Holthoff and Dimitra Stratis-Cullum

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14. ABSTRACT

Molecularly imprinted polymers (MIPs) can be used as artificial recognition elements for target chemical analytes of interest. 

Molecular imprinting involves arranging polymerizable functional monomers around a template followed by polymerization 

and template removal. The selectivity for the target analyte is based on the spatial orientation of the binding site and covalent 

or noncovalent interactions between the functional monomer and the analyte. In a sensor format, the recognition event is 

monitored with some form of transduction. MIP technology is still in its infancy and limitations such as non-specific binding 

may be overcome using surface enhanced Raman scattering (SERS) as an integrated transduction method for enhanced sensor 

performance. Compared to other spectroscopic techniques employed with MIPs, SERS should be less affected by the crossselectivity resulting from non-specific adsorption to the polymer. This report details the development of a MIP-SERS sensing 

platform for explosive targets.

15. SUBJECT TERMS 

Molecularly imprinted polymer (MIP), surface enhanced Raman scattering (SERS), explosives detection, Xerogel 

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Ellen Holthoff

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iii

Contents

List of Figures iv

Acknowledgments v

1. Objective 1

2. Approach 1

3. Results 4

3.1 TNT-doped Xerogel Preparation.....................................................................................4

3.2 Controlling Film Thickness.............................................................................................4

3.3 MIP Performance ............................................................................................................5

3.4 MIP Selectivity................................................................................................................7

3.5 MIP Stability ...................................................................................................................9

3.6 Colloid-embedded Xerogel SERS-active Substrates ....................................................10

4. Conclusions 11

5. References 12

6. Transitions 14

List of Symbols, Abbreviations, and Acronyms 15

Distribution List 16
iv

List of Figures

Figure 1. Proposed MIP-SERS concept..........................................................................................1

Figure 2. SERS spectra of a TNT-doped xerogel MIP deposited on Klarite substrates at 

different (a) volumes and (b) spinning speeds...........................................................................5

Figure 3. SERS spectra of (a) TNT-doped and control xerogel thin films and (b) a control 

subtracted TNT-doped xerogel thin film. ..................................................................................6

Figure 4. SERS spectra from a TNT-doped xerogel thin film recorded before and after the 

TNT was removed with the wash solution. ..............................................................................6

Figure 5. SERS spectra recorded after a MIP and controls A and B were incubated in a TNT 

solution; 85% of the TNT is removed by the wash step..........................................................7

Figure 6. SERS spectra (a) recorded after a MIP and controls A and B were incubated in a 

TNT solution and then rinsed with acetonitrile and (b) for a MIP and control A after 

control B subtraction..................................................................................................................7

Figure 7. SERS spectra recorded after a MIP and were incubated in a DNT solution and then 

rinsed with acetonitrile...............................................................................................................8

Figure 8. SERS spectra recorded after a MIP and control A were incubated in a NT solution 

and then rinsed with acetonitrile................................................................................................9

Figure 9. Assessment of TNT, DNT, and NT binding to the MIP in comparison to control 

A.................................................................................................................................................9

Figure 10. (a) TEM of silver nanoparticles coated with xerogel and (b) EDX analysis 

verifying the presence of silver................................................................................................10

Figure 11. SERS spectra for BTEB-coated silver and BTEB control nanoparticles...................11
v

Acknowledgments

We would like to thank Mikella Hankus for her assistance with the Raman measurements of the 

colloid-embedded structures and Dr. Wendy Sarney for her help with the transmission electron 

micrograph (TEM) analysis of the colloidal structures.
vi

INTENTIONALLY LEFT BLANK.
1

1. Objective

The objective of this Director’s Research Initiative (DRI) is to investigate, for the first time, the 

feasibility of using molecularly imprinted polymers (MIPs) and surface enhanced Raman 

scattering (SERS) to develop highly selective and sensitive nanosensors for detecting explosive 

compounds.

2. Approach

The Army does not have a sensing technology that meets both the chemical selectivity and 

sensitivity requirements for detection of a wide range of explosive compounds. Our approach 

aims to address this significant shortfall through the combination of two separate technologies, 

MIPs and SERS. Although each technology has been separately studied in the literature with 

limited success, this report is the first successful integration and demonstration of this multifunctional approach for explosives detection to date. In this first-year effort, we have clearly 

validated the MIP-SERS concept to include the development of an integration strategy and MIP 

formulation that has excellent adhesion properties, along with a demonstration of integrated 

SERS detection. Although not planned in the first-year objectives, we have demonstrated a 

greater selectivity of the imprinted polymer for 2,4,6-trinitrotoluene (TNT) over other nitrocontaining compounds. Initially, we chose a commercially available SERS substrate platform to 

validate the sensing concept; however, we aim to address the required sensitivity through more 

advanced substrate development and integration approaches as the project continues. 

MIPs can be used as artificial recognition elements for target chemical analytes of interest. 

Essentially, one creates a molecular “memory” within the imprinted polymer matrix. In a sensor 

format, the recognition event is monitored with some form of transduction (i.e., optical, 

electrochemical, etc.). MIP technology is still in its infancy and limitations such as non-specific 

binding may be overcome by using SERS as an integrated transduction method for enhanced 

sensor performance. The basic MIP-SERS concept is illustrated in figure 1.

Template

molecule

Polymerizable

functional monomer

Polymerization

Formation of 

imprinted sites

Removal of

template molecule

Unoccupied 

template site

Target analyte

binding

Polymer matrix

Assembly

Addition of SERSactive nanoparticles

SERS-active

nanoparticle

SERS 

detection

Wavenumber (cm-1)

Intensity (a.u.)

Template

molecule

Polymerizable

functional monomer

Polymerization

Formation of 

imprinted sites

Removal of

template molecule

Unoccupied 

template site

Target analyte

binding

Polymer matrix

Assembly

Addition of SERSactive nanoparticles

SERS-active

nanoparticle

SERS 

detection

Wavenumber (cm-1)

Intensity (a.u.)

Wavenumber (cm-1)

Intensity (a.u.)

Figure 1. Proposed MIP-SERS concept.
2

Briefly, MIPs involve arranging polymerizable functional monomers around a template,

followed by polymerization and template removal (1). Arrangement is generally achieved by

non-covalent or reversible covalent interactions. In both types of molecular imprinting, once the 

template is removed, three-dimensional cavities are generated within the final material that are 

complementary to the template molecule in size, shape, and functionality. This arrangement 

allows the preparation of polymers that are selective for the adsorption of the target molecule of 

interest. Other advantages of this technique include robustness and stability under a wide range 

of chemical and physical conditions, and an ability to easily design recognition sites for a 

plethora of target chemicals (e.g., pesticides, energetic materials, pharmaceuticals, and proteins)

(2, 3). There have been some published work and patents on the development of MIP materials 

for explosive compounds (3–5). However, a MIP alone does not meet the requirements for a 

sensor. Thus, unless additional selectivity is achieved through some other means, a MIP alone

will not be functional for practical application. 

The detection capabilities of SERS should make it an excellent transduction method for

selective, full compound identification, a capability that is not currently possible with existing 

MIP sensors. SERS is an extremely sensitive and selective technique that involves enhancements 

in the Raman scattering intensities of substances adsorbed on a rough metal surface (typically,

gold or silver). The enhancements are due to an increase in the electric field surrounding the 

molecules, which results when the incident light in the experiment strikes the metal surface and 

excites localized surface plasmons. The enhancement factor can be as great at 1014, which 

permits single molecule detection (6). Compared to other, more conventional spectroscopic 

techniques employed with MIPs, SERS should be less affected by the cross-selectivity resulting 

from non-specific adsorption to the polymer. As part of our technique, the MIP will concentrate 

the target onto the SERS-active surface, thereby making the combined approach more selective 

than a SERS-only detection platform, which is plagued by background chemical interference.

This integrated MIPs and SERS concept could be a very attractive approach as a chemical 

sensing platform for the Army, yet only two investigations of this pairing have been reported

(7, 8). In the first, before the onset of this project, Kostrewa et al. (7) prepared MIPs on SERSactive surfaces to directly monitor the uptake and release of either (2S,3S)-(+)-di-O-benzoyltartaric acid or N-benzyloxycarbonyl-(L)-apartic acid to a polymer by SERS. Adhesion of the 

MIPs to the SERS-active substrates was unsatisfactory for practical application as the MIP was 

not truly integrated with the substrate. In our approach, we have incorporated a specific 

functional group in the chemical formulation that binds to the metallic under layer support 

through thiol chemistry. During this year, the Kantarovich group (8) used a nano fountain pen to 

print MIP droplets on SERS-active surfaces and directly monitor the uptake and release of the βblocking drug propranolol by SERS. In both of these experiments, the MIP was not designed for 

explosive targets and the SERS-active metallic nanostructures were not integrated into the 

polymer imprint, which is key to the development of a sensitive and robust nanosensor platform. 
3

We also conducted a literature and patent search using Web of Science and SciFinder. In 

summary, the survey revealed literature pertaining to MIPs for explosive compounds (3–5) or 

SERS for explosives recognition (9–12), but there were no reports on an integrated MIP-SERS 

nanosensor for explosives detection. Also, it is worth pointing out that there are many groups 

and companies (e.g., Edmiston, EIC Labs) that are pursuing SERS-only sensing platforms (with 

no MIP capability) or MIP-based schemes with other transduction methods.

The goal of the current work was to demonstrate that the MIP-SERS combination is an effective 

and robust chemical nanosensing scheme and the first-ever hybrid MIP-SERS sensing platform 

for explosive targets. The feasibility of the hybrid MIP-SERS nanosensing approach was

assessed through polymer optimization for explosive imprinting, enhanced polymer adhesion to 

the substrate, and integration with SERS through active nanoparticle templating and 

commercially available SERS substrates.

Polymer optimization involved combining polymers in a precise manner so as to create a variety 

of usable polymer materials. The polymer materials of interest are sol-gel-derived xerogels, 

which have been used as a platform for MIP-based sensor development (13–15). The sol-gel 

process is favorable as it allows one to entrap analytes within a readily tunable material (16).

These materials are also attractive because their physicochemical properties can be adjusted by 

choice of precursor(s) and the processing protocol. Xerogel hosts may be prepared with (1) a 

wide range of surface areas, mean pore dimensions, and pore size distributions; (2) good thermal 

stability; (3) adjustable conductivities; and (4) a reasonably broad optical window that allows the 

use of modern spectroscopic tools to study dopants within the xerogel. Precursors chosen based 

on potential interactions with the template molecule (and target analyte), TNT, allow for 

increased target recognition. For example, MIPs comprised of 3-aminopropyltriethoxysilane, 

2-(trimethoxysilylethyl)pyridine or acrylamide have been shown to engage in strong noncovalent interactions with TNT molecules (17) via the formation of a charge-transfer complex 

between the electron-deficient aromatic ring of nitroaromatics and the electron-rich amino or 

pyridine group of the precursor (18–20). This interaction significantly improves polymer 

selectivity and affinity for TNT (3). Adhesion of the polymer to the SERS substrate is also a 

consideration in precursor selection. For example, chemisorption of the polymer to the metal 

may be achieved by incorporating a xerogel precursor that contains a thiol or disulfide group, 

such as 2-(2-pyridylethyl)thiopropyltrimethoxysilane or bis[3-(triethoxysilyl)propyl]disulfide.

Although MIPs have been integrated with other, less selective spectroscopic methods such as 

fluorescence, integration with surface enhanced Raman is not straightforward and poses a unique 

set of challenges. One challenge, discussed previously, is successful polymer integration, which 

we address through thiol precursor chemisorption. However, a major technical challenge is 

ensuring that the polymer layer is a thin enough film to concentrate the chemical target of 

interest (in this case, an explosive compound) within the surface plasmon responsible for the 

Raman enhancement, which drops off exponentially with distance from the nanostructure 

surface. Therefore, thin-film formulations are necessary (sub micron) with our casting approach. 
4

In order to validate this concept in the first year, our first approach was to use commercially 

available SERS-active substrates (KlariteTM) and thin-film MIP integration. These substrates 

offer reproducibility, but lack the required sensitivity. The longer-term approach was to 

integrate the MIP with more advanced substrate architectures, such as a multi-layer film over 

nanosphere approach (21, 22) or colloidal encapsulation. Therefore, the sensitivity of the sensor 

for TNT would be limited by the SERS-active substrate. Although outside the first-year 

objectives, SERS-active substrates were prepared in-house by embedding metal colloids (silver 

or gold) in a sol-gel-derived structure (23). This approach combined the high activity of colloids 

with the stability of xerogels. Sol-gel-based molecularly imprinted SERS substrates may be

prepared by embedding silver or gold colloids in the MIPs. 

3. Results

3.1 TNT-doped Xerogel Preparation

We have developed a procedure for polymer fabrication and formulated a sol-gel-derived xerogel 

following this process. A TNT stock solution was prepared at 6.05 x 10–3 M in acetonitrile. A 

high concentration of TNT was needed due to the sensitivity limits of the Klarite substrates. Sol 

solutions were prepared by mixing methyltriethoxysilane (C1-TriEOS), 

3-mercaptopropyltrimethoxysilane (MPTMS), 3-aminopropyltriethoxysilane (APTES), ethanol 

(EtOH), and hydrochloride (HCl) (1 M). These components were simultaneously combined at 

room temperature and then stirred for 30 min to ensure a visually homogeneous sol solution. 

The TNT-doped sol solution was prepared by adding 150 μL of the TNT stock solution to the 

prehydrolyzed C1-TriEOS/MPTMS/APTES/EtOH/HCl sol solution. This solution was then 

vigorously mixed for 30 s with a vortex mixer. The strong non-covalent interaction between 

APTES and TNT resulted in the appearance of a red color. Control xerogel films were prepared 

to ensure that the observed SERS signal did not arise from artifacts. The controls were prepared 

by following the exact reaction sequence described above except as noted in section 3.2. Control 

A was formed by eliminating TNT and control B was formed by eliminating TNT and APTES.

3.2 Controlling Film Thickness

The xerogels were spun cast as thin films onto purchased Klarite SERS substrates using a spin 

coater. SERS enhancement is known to decrease tenfold with a distance of approximately 

2–3 nm. Due to the short range of this phenomenon, the templated sites of the polymer must be 

kept within the enhancing electromagnetic fields; therefore, the thickness of the MIP films 

deposited on the substrates for SERS detection must be controlled. In order to determine the 

optimal thickness, the volume and spin deposition rate of the sol solutions was optimized. 

Figure 2a presents SERS spectra collected for three volumes (50, 75, and 100 μL) of a TNTdoped xerogel MIP deposited by spin casting (2000 rpm, 30 s) on Klarite substrates. Plainly 
5

apparent in all the samples are the nitrogen dioxide (NO2) out-of-plane bending modes between 

820 and 850 cm–1 and the NO2 stretching modes between 1350 and 1370 cm–1 (24). These bands 

can be used as a “fingerprint” for the detection of TNT by its SERS spectrum. Comparison of 

the spectra for each sample volume suggests there is less polymer background when smaller 

aliquots of sample are used. Although SERS detection was achieved for all volumes of the TNTdoped MIP, the films ranged in thickness from 7–10 μm, which is outside of the enhancing 

electromagnetic field. We suspect a high concentration of TNT resulted in a majority of the 

signal arising from the substrate-film interface.

Figure 2b presents SERS spectra collected for 50 μL of a TNT-doped xerogel MIP deposited by 

spin casting on Klarite substrates at three spinning speeds (2000, 4000, and 6000 rpm). The 

TNT fingerprint is apparent in these spectra. Comparison of the spectra at each spinning speed 

suggests there is less polymer background at faster speeds. Spin casting at 4000 and 6000 rpm 

yielded films ranging in thickness from 2–5 μm. Based on the results from the volume and 

speed studies, we determined the optimal parameters for sample preparation to be 50 μL of 

sample, spun cast at 4000 rpm. 

Figure 2. SERS spectra of a TNT-doped xerogel MIP deposited on Klarite substrates at different (a) volumes and 

(b) spinning speeds.

3.3 MIP Performance

Figure 3a presents the SERS spectra for both a TNT-doped xerogel thin film and control (A and 

B) xerogel thin films. The TNT fingerprint is apparent in the spectra of the doped samples. The 

control spectra are similar and have only a broad peak between 1300 and 1340 cm–1. This peak 

is also partially apparent in the doped samples. This is attributed to the glass to which the Klarite

active area is mounted. A glass spectrum is provided in figure 3a for comparison. Figure 3b

presents the SERS spectra for a TNT-doped xerogel thin film with the control spectra subtracted. 

The broad glass background peak is no longer evident and the TNT fingerprint peaks are more 

defined.

Raman Shift (cm-1)

600 800 1000 1200 1400 1600

SERS Intensity (a.u.)

12000

14000

16000

18000

20000

22000

24000

26000

TNT 50μL

TNT 75μL 

TNT 100μL 

Raman Shift (cm-1)

600 800 1000 1200 1400 1600

SERS Intensity (a.u.) 15000

20000

25000

30000

35000

40000

2000 RPM

4000 RPM

6000 RPM

A B
6

Raman Shift (cm-1)

800 1000 1200 1400 1600

SERS Intensity (a.u.) 3000

4000

5000

6000 TNT corrected

1000 μg/mL TNT on KlariteTM

Raman Shift (cm-1)

800 1000 1200 1400 1600

SERS Intensity (a.u.)

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

TNT-doped

Glass

Control A

Control B

A B

Figure 3. SERS spectra of (a) TNT-doped and control xerogel thin films and (b) a control subtracted TNT-doped 

xerogel thin film.

Xerogel thin films were treated with an EtOH/acetonitrile/acetic acid (v/v/v 8:2:1) solution at 

room temperature. The xerogels were allowed to react with the EtOH/acetonitrile/acetic acid 

solution overnight (18 h). Figure 4 presents the SERS spectra for a TNT-doped xerogel thin film 

before and after treatment with the EtOH/acetonitrile/acetic acid solution. The results of this 

experiment suggest that 85% of the TNT is removed by the wash step. 

Raman Shift (cm-1)

800 1000 1200 1400 1600

SERS Intensity (a.u.) 1000

2000

3000

4000

5000

6000 Before wash

After wash

Figure 4. SERS spectra from a TNT-doped xerogel thin film 

recorded before and after the TNT was removed 

with the wash solution.

A TNT stock solution was prepared at 0.4 mM in acetonitrile. MIP and control thin films were 

incubated in this stock solution at room temperature. The xerogels were allowed to react with 

the TNT for 1.5 h. Figure 5 presents the SERS spectra for a MIP xerogel thin film and controls 

A and B after treatment with the TNT solution. The TNT fingerprint is apparent in all of the 

spectra. The results of this experiment suggest that TNT remains on the surface of the xerogel 

thin films. The MIP and controls were rinsed with 2 mL of acetonitrile in order to remove the 

TNT from the film surface. Figure 6a presents the SERs spectra for a MIP xerogel thin film and 

controls A and B after the acetonitrile rinse. There was no longer a TNT fingerprint present in
7

the SERS spectrum collected for control B. The control B SERS spectrum was subtracted from 

the SERS spectra for a MIP xerogel thin film and control A. These spectra are shown in figure 

6b. The results of this experiment suggest that the MIP binds 67% more TNT than control A and 

100% more TNT than control B. It was expected that some of the TNT would react with control 

A due to the presence of APTES.

Raman Shift (cm-1)

800 1000 1200 1400 1600

SERS Intensity (a.u.)

4000

6000

8000

10000

12000

14000 MIP

Control A

Control B

Figure 5. SERS spectra recorded after a MIP and controls A 

and B were incubated in a TNT solution; 85% of 

the TNT is removed by the wash step.

Figure 6. SERS spectra (a) recorded after a MIP and controls A and B were incubated in a TNT solution and then 

rinsed with acetonitrile and (b) for a MIP and control A after control B subtraction.

3.4 MIP Selectivity

The MIP was challenged by two structurally similar analogues to TNT. A 2,4-dinitrotoluene 

(DNT) stock solution was prepared at 0.4 mM in acetonitrile. MIP and control A thin films were 

incubated in this stock solution at room temperature. The xerogels were allowed to react with 

the DNT for 1.5 h. Figure 7 presents the SERS spectra for a MIP xerogel thin film and control A 

after treatment with the DNT solution and rinsing with 2 mL acetonitrile. Similar to TNT, DNT 

also exhibits NO2 out-of-plane bending modes between 820 and 850 cm–1 and the NO2 stretching 

Raman Shift (cm -1)

800 1000 1200 1400

SERS Intensity (a.u.)

0

500

1000

1500

2000 MIP

Control A

Raman Shift (cm -1)

800 1000 1200 1400 1600

SERS Intensity (a.u.)

4000

6000

8000

10000

12000 MIP

Control A

Control B

A B
8

modes between 1350 and 1370 cm–1 (11). DNT also exhibits the aromatic ring breathing mode 

around 1000 cm–1. The results of this experiment suggest that the MIP and control A bind 

similar amounts of DNT. The MIP binds only 6% more DNT than control A. Based on the 

presence of APTES, it was expected that some of the DNT would react with the MIP and control 

A. Because DNT is a precursor and a manufacturing impurity of TNT, it is possible that DNT 

was the template molecule for some of the imprinted sites in the MIP, which explains the 

minimal binding. Also, due to the size and shape of the templated sites in relation to DNT, it is 

possible that some of the DNT was bound in these sites.

Raman Shift (cm-1)

800 1000 1200 1400 1600

SERS Intensity (a.u.)

60000

80000

100000

120000

140000

160000

180000

200000

MIP

Control A

1000 μg/mL DNT 

on KlartieTM

Figure 7. SERS spectra recorded after a MIP and were incubated 

in a DNT solution and then rinsed with acetonitrile.

A 2-nitrotoluene (NT) stock solution was prepared at 0.4 mM in acetonitrile. MIP and control A 

thin films were incubated in this stock solution at room temperature. The xerogels were allowed 

to react with the NT for 1.5 h. Figure 8 presents the SERS spectra for a MIP xerogel thin film 

and control A after treatment with the NT solution and rinsing with 2 mL acetonitrile. Similar to 

TNT and DNT, NT also exhibits the NO2 stretching mode between 1350 and 1370 cm–1, 

although it is not as pronounced. The aromatic ring breathing mode around 1000 cm–1 is also 

present. The results of this experiment suggest that the MIP and control A bind minimal 

amounts of NT and the MIP binds 5% more NT than control A. Again, based on the presence of 

APTES, it was expected that some of the NT would react with the MIP and control A. Due to 

the size and shape of the templated sites in relation to NT, it is possible that some of the NT was 

bound in these sites, which explains the minimal binding.
9

Raman Shift (cm-1)

800 1000 1200 1400 1600

SERS Intensity (a.u.) 50000

60000

70000

80000

90000

MIP

Control A

1000 μg/mL NT on KlariteTM

Figure 8. SERS spectra recorded after a MIP and control A 

were incubated in a NT solution and then rinsed

with acetonitrile.

Figure 9 presents a bar graph comparing the percentage of TNT, DNT, and NT binding to the 

MIP in comparison to control A. The percentage of MIP binding is significantly greater for TNT 

and suggests a successful preliminary demonstration of a MIP-SERS sensing platform for this 

explosive target.

TNT DNT NT

% MIP Binding

0

20

40

60

80

Figure 9. Assessment of TNT, DNT, and NT binding to the 

MIP in comparison to control A.

3.5 MIP Stability

During our effort, we overcame the technical barriers to MIP adhesion. This was achieved by 

incorporating the xerogel precursor, MPTMS, in the xerogel formulation used to prepare the 

MIP. The thiol group in this precursor chemisorbs onto the metal surface of the Klarite 

substrate. Adhesion was assessed by soaking the xerogel-coated substrate in various aqueous 

environments, including water (H2O), 6 M HCl, 0.1 M pH 7.4 phosphate buffer, and the

ethanol/acetonitrile/acetic acid wash solution. The polymer showed excellent adhesion and 

stability, with no apparent degradation, which is necessary for practical field use.
10

3.6 Colloid-embedded Xerogel SERS-active Substrates

We also developed a procedure for coating silver nanoparticles with a xerogel. Preparation of 

silver nanoparticle suspensions has been reported elsewhere (25). To coat the particles, 1 mL of 

the stock silver suspension was first diluted with 200 mL 2-propanol with stirring. To this 

solution we added 10.5 mL ammonium hydroxide (NH4OH) and 18.0 mL distilled H2O. This 

mixture was stirred at 600 rpm at 40 oC. After the temperature was stabilized, a mixture of 

xerogel precursors, tetraethoxysilane (TEOS), 0.101 mL, and 

1,4-bis(trimethoxysilylethyl)benzene (BTEB) , 0.0394 mL, were rapidly injected and the reaction 

continued for 2 h. The xerogel shell thickness varies depending on the amount of precursor and 

the reaction time. Specific conditions have been reported elsewhere (26). Control nanoparticles 

were prepared by eliminating the silver nanoparticle suspension. 

Figure 10a presents a transmission electron micrograph (TEM) of silver nanoparticles coated 

with xerogel. The dark silver centers can be seen encapsulated with approximately 50 nm of 

material. Figure 10b is the energy dispersive x-ray (EDX) analysis of the nanoparticles. A 

prominent silver peak is observed, verifying the presence of silver.

Figure 10. (a) TEM of silver nanoparticles coated with xerogel and (b) EDX analysis verifying the presence of 

silver.

Figure 11 presents the SERS spectra for xerogel-coated silver nanoparticle and control

nanoparticles. The spectrum for BTEB on a Klarite substrate is also shown for reference. In the 

reference spectrum, there are two prominent peaks. The peaks around 700 and 1100 cm–1 can be 

used as a fingerprint for BTEB. This BTEB fingerprint is visible in the spectrum for the BTEBcoated silver nanoparticles. The results of this experiment suggest that SERS-active substrates 

were successfully prepared by embedding metal colloids in a xerogel structure. It is expected 

that controlling shell thickness will provide increased SERS enhancement.
11

Raman Shift (cm-1)

600 800 1000 1200

SERS Intensity (a.u.)

0

1000

2000

3000

4000

5000

BTEB-coated Ag nanoparticles

Control BTEB nanoparticles

BTEB on KlariteTM

Figure 11. SERS spectra for BTEB-coated silver and BTEB 

control nanoparticles.

4. Conclusions

The work reported here validates the hybrid MIP and SERS sensing concept and demonstrates 

the feasibility of this nanosensor concept for the detection of explosive compounds. To date, this 

is the only demonstration of a MIP-SERS sensing platform for these targets. Although 

preliminary, the data suggests that the MIP-SERS combination is an effective and robust 

chemical nanosensing scheme. Although high-risk, the successful development of a MIP-SERS 

sensing platform for direct target analyte detection would be high impact as it could provide the 

Army with an adaptable chemical nanosensing platform for rapid and robust detection of a 

variety of threat and target species. 

Further investigations should focus on refinement of both the MIP (i.e., xerogel formulation) for 

improved selectivity and the SERS substrate for enhanced sensitivity. Also, the MIP-SERS 

sensor platform evaluation should be expanded to include other explosives and chemical warfare 

agents of interest to the Army. This research could provide a base for an effort at the U.S. Army 

Research Laboratory (ARL) to develop MIPs suitable for the detection of a variety of chemical 

and biological agents. A successful MIP-SERS sensing format could reduce sensor cost and 

size, while maintaining the high sensitivity, selectivity, and portability needed for military 

applications. 
12

5. References

1. Yan, M.; Ramstrom, O. Molecularly Imprinted Materials: Science and Technology; Marcel 

Dekker: New York, NY, 2005.

2. Holthoff, E. L.; Bright, F. V. Anal. Chim. Acta 2007, 594, 147–161.

3. Walker, N. R.; Linman, M. J.; Timmers, M. M.; Dean, S. L.; Burkett, C. M.; Lloyd, J. A.; 

Keelor, J. D.; Baughman, B. M.; Edmiston, P. L. Anal. Chim. Acta 2007, 593, 82–91.

4. Gao, D.; Zhang, Z.; Wu, M.; Xie, C.; Guan, G.; Wang, D. J. Am. Chem. Soc. 2007, 129, 

7859–7866.

5. Xie, C.; Liu, B.; Wang, Z.; Gao, D.; Guan, G.; Zhang, Z. Anal. Chem. 2008, 80, 437–443.

6. Fleischmann, M.; Hendra, P. J.; Quillan, A.J.M. Chem. Phys. Lett. 1974, 26, 163–166.

7. Kostrewa, S.; Emgenbroich, M.; Klockow, D.; Wulff, G. Macromol. Chem. Phys. 2003, 204, 

481–487.

8. Kantarovich, K.; Tsarfati, I.; Gheber, L.; Haupt, K.; Bar, I. Writing Droplets of Molecularly 

Imprinted Polymers by Nano Fountain Pen and Detecting Their Molecular Interactions by 

Surface-Enhanced Raman Scattering. Anal. Chem. 2009, 81, 5686–5690.

9. Kneipp, K.; Wang, Y.; Dasari, R. R.; Feld, M. S.; Gilbert, B. D.; Janni, J. A.; Steinfeld, J. I.

Spectrochim. Acta A 1995, 51, 2171–2175.

10. Spencer, K. M.; Sylvia, J. M.; Marren, P. J.; Bertone, J. F.; Christensen, S. D. Proc. SPIE 

2004, 5269, 1–8.

11. Sylvia, J. M.; Janni, J. A.; Klein, J. D.; Spencer, K. M. Anal. Chem. 2000, 72, 5834–5840.

12. Spencer, K. M.; Sylvia, J. M.; Janni, J. A.; Klein, J. D. Proc. SPIE 1999, 3710, 373–379.

13. Graham, A. L.; Carlson, C. A.; Edmiston, P. L. Anal. Chem. 2002, 74, 458–467.

14. Leung, M.K.P.; Chow, C.-F.; Lam, M. H. W. J. Mater. Chem. 2001, 11, 2985–2991.

15. Shughart, E. L.; Ahsan, K.; Detty, M. R.; Bright, F. V. Anal. Chem. 2006, 78, 3165–3170.

16. Brinker, C. J.; Scherer, G. W. Sol-Gel Science; Academic Press: New York, NY, 1989.

17. Xie, C.; Zhang, Z.; Wang, D.; Guan, G.; Gao, D.; Liu, J. Anal. Chem. 2006, 78, 8339–8346.

18. Kang, S.; Green, J. P. Proc. Natl. Acad. Sci. U.S.A. 1970, 67, 62–67.

19. Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321–5322.
13

20. Rose, A.; Zhu, Z.; Madigan, C. F.; Swager, T. M.; Bulovic, V. Nature 2005, 434, 876–879.

21. Li, H. G.; Baum, C. E.; Sun, J.; Cullum, B. M. Appl. Spectrosc. 2006, 60, 1377–1385.

22. Li, H. G.; Cullum, B. M. Appl. Spectrosc. 2005, 59, 410–417.

23. Akbarian, F.; Dunn, B. S.; Zink, J. I. J. Raman Spectrosc. 1996, 27, 775–783.

24. Jerez Rozo, J. I.; del Rocio Balaguera, M.; Cabanzo, A.; de la Cruz Montoya, E.; HernandezRivera, S. P. Proc. SPIE 2006, 6201, 62012G-1-62012G-8.

25. Guicheteau, J. A.; Christesen, S. D. Proc. SPIE 2004, 5585, 113–121.

26. Hardikar, V. V.; Matijevic, E. J. Colloid Interface Sci. 2000, 221, 133–136.
14

6. Transitions

This work was presented at the 238th American Chemical Society National Meeting and 

Exposition, August 16–20, 2009, in Washington, DC. A paper for publication is in preparation 

and will be submitted in early 2010. 
15

List of Symbols, Abbreviations, and Acronyms

APTES 3-aminopropyltriethoxysilane 

ARL U.S. Army Research Laboratory 

BTEB 1,4-bis(trimethoxysilylethyl)benzene 

C1-TriEOS methyltriethoxysilane 

DNT 2,4-dinitrotoluene 

DRI Director’s Research Initiative 

EDX energy dispersive x-ray 

EtOH ethanol 

H2O water 

HCl hydrochloride

MIPs molecularly imprinted polymers

MPTMS 3-mercaptopropyltrimethoxysilane 

NH4OH ammonium hydroxide 

NO2 nitrogen dioxide 

NT 2-nitrotoluene 

SERS surface enhanced Raman scattering 

TEM transmission electron micrograph 

TEOS tetraethoxysilane

TNT 2,4,6-trinitrotoluene
16

No. of

Copies Organization

1 ADMNSTR

ELEC DEFNS TECHL INFO CTR

ATTN DTIC OCP

8725 JOHN J KINGMAN RD STE 0944

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1 CD US ARMY RSRCH LAB

ATTN RDRL CIM G T LANDFRIED

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21005-5066

3 CDS US ARMY RSRCH LAB

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MAIL & RECORDS MGMT

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