Supramolecular Functionalised Optical Surfaces For Remote Sensing

NH2

R

N+

N

R

Cl

NN

O

OH

OR

NN

O

Cl

OR

OH

Br

O

O

NN

OHR

NN

O

O

OR

NaNO2

HCl

0-5oC 0-5oC

SOCl2

reflux

K2CO3

NaIAcetone

NaOH (aq)

EtOH

4

23

1

Ra = -HRb = -NO2

Rc = -OMe

OSi

NH2

OO Si NH2

OEt

EtO OEt

NN

O

O

NH

OSi OO

O

NN

O

O Cl

O

TEA, THF

SiO2 / Ta2O5

1) Piranha solution2) NH4OH/H2O2/H2O3)EtOH/H2O,

SiO2 / Ta2O5 SiO2 / Ta2O5

OSi

SH

OO Si SH

OMe

OMe

MeO

S

OSi OO

SiO2/Ta2O5

1) Piranha solution2) NH4OH:H2O2:H2O3) iPrOH/H2O

SiO2/Ta2O5 SiO2/Ta2O5

Au

Au

r2

r1

s

s

s

r2

r2

r1

r1

10

1.5

2 21 5

10

20 mm

NN

R

OH

O

N

N

R

O OH

hv (350 nm)

hv (525 nm) (or heat)

trans-isomer cis-isomer

Rc = -OMe

Ra = -HRb = -NO2

An exposed Bragg grating, incorporated into a planar waveguide is used to form an optical device that acts as a refractive index sensor. The exposed region makes the Bragg peak sensitive to the refractive index of its surroundings. The corresponding peak wavelength shift can be used to detect changes in this environment. Functionalising the surface allows this shift to be directed towards specific interactions, allowing for the creation of a wide range of optical devices, including: sensors, switches and modulators.

The device has two key areas; the first is a buried reference Bragg grating {r1} that will sense thermal changes. The waveguide then splits through a “y-splitter” to give a sensor grating {s} and an exposed reference grating {r2}.

Removal of the silica overclad exposes the Bragg gratings, allowing the device to sense changes in refractive index. However the low refractive index above the sensor surface results in asymmetry in the guided mode (Figure 5.a.). This shifts the mode further away from the surface, reducing sensitivity. Tantalum pentoxide has a significantly higher refractive index (~2.08) than that of silica (~1.44). A thin layer of tantalum pentoxide restores the guided mode, vastly increasing the sensitivity (Figure 5.b.):

Tantalum pentoxide was sputtered directly onto freshly cleaned samples to a depth of 80-100 nm. The change in surface sensitivity was assessed through calibration against a series of oils of known refractive index both before, and after the deposition of tantalum pentoxide. Both Bragg gratings present within the well were affected by the presence of the oil on the surface, with the peak wavelength shifting by up to 1 nm (Figure 6). However when the same waveguides were coated with 80.8 nm of tantalum pentoxide the peak wavelength shift dramatically increased by up to an order of magnitude at higher refractive indices.

Figure 6 – the improvement in the peak Bragg wavelength shift upon exposure to oils with

refractive index of 1.30 - 1.45: before (blue) and after (red) coating with 80.8nm of tantalum

pentoxide .

References – 1) I. Sparrow, PhD thesis, University of Southampton, 2005. 2) R. D. Palma, W. Laureyn, F. Frederix, K. Bonroy, J.-J. Pireaux, G. Borghs and G. Maes, Langmuir, 2007, 23, 443-451. 3) C. A. Goss, D. H. Charych and M. Majda, Anal. Chem., 1991, 63, 85-88. 4) S. H. Barley, A. Gilbert and g. R. Mitchell, J. Mater. Chem., 1991, 1, 481-482.

R. M. Parker†, J. C. Gates‡, P. G. R. Smith‡, M. C. Grossel†,†School of Chemistry and the ‡Optoelectronics Research Centre, University of Southampton, United Kingdom, rmp203@soton.ac.uk.

A well-known property of azobenzene compounds is their ability to selectively isomerise from the cis- to trans- isomer upon exposure to light. On undergoing photoisomerisation, there is a bulk change in refractive index4, this property can be exploited to produce an optical device. A family of para- substituted azobenzene derivatives were synthesised (Figure 8):

The solution phase UV-vis absorption spectra for compounds 3a, 3b and 3c are shown below (Figure 9) for a concentration of 2.5 x10-5 mol dm-3 in methanol. For all three compounds, exposing the trans- isomer to 365 nm light induces photoisomerism to the cis- isomer, with a reduction of the peak at 340-370 nm. 3c has the largest absorption transition corresponding to the highest electron density within the azobenzene core.

Addition of (3-aminopropyl)triethoxysilane to the tantalum pentoxide surface resulted in a shift in peak wavelength of 46 (±3) pm, with a further shift of 22 (±1) pm on addition of 4c (Figure 10). Both shifts are significantly above the level of thermal noise of the system demonstrating that the optical system is capable of sensing changes occurring on the surface.

Figure 5 – the enhanced symmetry of the guided mode travelling along an exposed waveguide with an overlayer of tantalum pentoxide (b) compared

to without (a).

It has been reported2 that tantalum pentoxide can be functionalised analogously to silica. This was tested by tethering a layer of (3-mercaptopropyl)trimethoxysilane to the surface and then subsequently depositing 50 nm of gold3. Gold is known to tether to thiols, but will not adhere strongly to silica or tantalum pentoxide surfaces, allowing functionalised and unfunctionalised areas to be differentiated. Adhesion tests demonstrated that untreated surfaces retained no gold, while both silica and tantalum pentoxide functionalised surfaces retained complete gold films.

Figure 9 – absorption spectra for cis- (dashed) and trans- (solid) para-substituted azobenzene derivatives.

Figure 7 – successful tethering of gold to a tantalum pentoxide surface via an organic linker.

Figure 8 – the synthetic pathway towards a range of para-substituted azobenzene derivatives.

Green: cis-3a (λmax = 320.5, 450.5 nm) trans-3a (λmax = 342.0 nm)

Red: cis-3b (λmax = 367.5 nm) trans-3b (λmax = 370.0 nm)

Blue: cis-3c (λmax = 444.0 nm) trans-3c (λmax = 356.0 nm)

Figure 10 – functionalisation of a tantalum pentoxide surface with a substituted azobenzene unit.

Au (SiO2)

Au (Ta2O5)

Figure 4 – schematic of the characterisation setup (left) and the laboratory setup using standard telecomm measurement equipment (right).

Figure 2 – schematic of direct UV writing of planar waveguides containing Bragg gratings into germanium doped silica wafers (left); and photograph of direct UV writing (right).

Figure 3 – schematic of the UV written “y-splitter” optical device incorporating an etched well (purple); λs=1540 nm, λr1=1530 nm, λr2=1550 nm.

Figure 1 – schematic of an etched optical sensor, showing the exposed Bragg grating within the well.

An ASE (amplified stimulated emission) source was used to analyse the device. The output was polarised, before transmitting through a circulator to the device. The reflected light was directed to the optical spectrum analyser (OSA) which was remotely analysed via Labview to monitor the Bragg wavelength. The transverse magnetic (TM) mode was selected as it has greater surface penetration than the TE mode.

Planar waveguides can be written with a UV-laser into photosensitised silica to produce a wide range of optical devices. However when two beams are focused as to produce an interference pattern, Bragg gratings can additionally be written through careful modulation of the beam1.

Summary•Tantalum pentoxide, which increases the sensitivity of the optical device can be incorporated and functionalised analogously to silica.

•The successful synthesis and analysis of the absorption spectra of a family of azobenzene compounds has been achieved.

•The combination of the optical device design, with the introduction of tantalum pentoxide and the choice of chemistry has led to well resolved shifts in peak Bragg wavelength with surface functionalisation.