Synthesis of three-dimensional flower-like BiOCl:RE3+ (RE3+ = Eu3+, Sm3+) globular microarchitectures and their luminescence properties

Accepted Manuscript Title: Synthesis of three-dimensional flower-like BiOCl: RE3+ (RE3+ = Eu3+ , Sm3+ ) globular microarchitectures and their luminescence properties Author: Yang-Yang Guo Zhi-Jun Zhang Gang-Qiang Zhu Woochul Yang PII: DOI: Reference:

S0169-4332(15)03190-6 http://dx.doi.org/doi:10.1016/j.apsusc.2015.12.185 APSUSC 32168

To appear in:

APSUSC

Received date: Revised date: Accepted date:

13-10-2015 21-12-2015 22-12-2015

Please cite this article as: Y.-Y. Guo, Z.-J. Zhang, G.-Q. Zhu, W. Yang, Synthesis of three-dimensional flower-like BiOCl: RE3+ (RE3+ = Eu3+ , Sm3+ ) globular microarchitectures and their luminescence properties, Applied Surface Science (2015), http://dx.doi.org/10.1016/j.apsusc.2015.12.185 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis of three-dimensional flower-like BiOCl: RE3+ (RE3+ = Eu3+, Sm3+) globular microarchitectures and their luminescence properties

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Yang-Yang Guo a, Zhi-Jun Zhang a, Gang-Qiang Zhu b, Woochul Yang a,*

* Corresponding author. Woochul Yang, Tel: +82-2-2290-1397, fax: +82-2-2260-8713. E-mail: [email protected]

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a

Department of Physics, Dongguk University, Seoul, 100715, Korea

Department of Physics, Shanxi Normal University, Xi’an, 710062,China

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b

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Three-dimensional

flower-like

Eu3+ and

Sm3+ -activated

BiOCl

globular

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microarchitectures have been synthesized.

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Highlights:

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2. Ostwald ripening and recrystallization are responsible for the growth mechanism of BiOCl microarchitectures.

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structures of the microarchitectures.

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3. Efficient red-emission from Eu3+:BiOCl is observed due to the well-crystallized

Abstract: Three-dimensional flower-like Eu3+ and Sm3+-activated BiOCl globular microarchitectures were synthesized by the solvothermal method employing urea as a dispersing agent for the first time. The crystal structure, morphologies and luminescence properties of Eu3+ and Sm3+ doped BiOCl have been systematically investigated by powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) and 2 Page 2 of 45

spectroscopy, respectively. The unit cell volumes show a nearly linear decrease by about 0.18 and 0.15 % with increasing Eu3+ and Sm3+ concentration up to 9 mol%, respectively. All of the prepared samples show flower-like globular microarchitectures with an average

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diameter about 3 to 5 μm with different Eu3+ and Sm3+ concentrations. Possible formation mechanism for the flower-like microarchitectures is proposed on the basis of time-

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dependent experiment. Both BiOCl: Eu3+ and BiOCl: Sm3+ samples show a strong red

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emission corresponding to the 5D0 → 7F4 transition (700 nm) of Eu3+ and 4G5/2 → 6H7/2 transition (600 nm) of Sm3+, respectively. This work sheds some light on the design and

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preparation of red-emitting phosphors with novel microstructures.

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Keywords: BiOCl, flower-like structure, luminescence, solvothermal

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1. Introduction The morphology and size of the inorganic functional materials exhibit a great [1-4]

. In case of

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influence on their optical, magnetic and electrochemical properties

phosphors, different morphologies and sizes may introduce various dispersibility, surface

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states, as well as emission wavelength. Thus, the preparation of phosphors with the controllable morphology and size is of great importance to develop future lighting or

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display devices. Moreover, three-dimensional (3D) flower-like micro/nanomaterials have

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attracted considerable attention due to their unique properties resulting from the nanobuilding blocks and their potential applications in photoelectric devices, catalysts

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and biosensors [5-8]. Thus, great efforts have been dedicated to explore new approaches for the fabrication of hierarchical micro/nanostructures in different systems.

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In the last decade, due to unique layered crystal structure, excellent electronic and

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optical properties, as well as high chemical stability, bismuth oxyhalides, BiOX (X = Cl,

[9]

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Br, I) have attracted great attention in the field of photocatalysis and photoluminescence . Among BiOX, BiOCl is a type of wide band gap semiconductor with an indirect band

gap of 3.5 eV, exhibiting excellent electronic and optical properties with many important and promising applications, such as phosphor, photocatalyst, gas sensor, magnetic material, and pigment in the cosmetic industry [10, 11]. Typical BiOCl presents tetragonal structure with [Cl-Bi-O-Cl] layers stacked one above the other by van der Waals interaction through Cl along c-axis. The layered structure induces electric field through polarization of atoms and orbitals in the space between these layers, which facilitates the effective separation of photo-generated electron-hole pairs

[3]

. Moreover, the electron-

hole recombination is prohibited due to the indirect band gap nature of BiOCl, in which 4 Page 4 of 45

the excited electron in the conduction band has to emit or absorb phonon before a hole in valence band is annihilated radiatively. Eu3+ and Sm3+-activated materials as red-emitting phosphors are always the focus owing to its characteristic peak emissions generated by [12]

. Highly selective transitions

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inner 4f orbitals, long lifetime and large Stokes shifts

between these f levels result in narrow line emissions. Significantly, the ionic radius and

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charge of Bi3+ are comparable with that of Eu3+ and Sm3+, leading to the possible

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substitution of Bi3+ by Eu3+ and Sm3+ in BiOCl lattice. As a result, BiOCl is a suitable host lattice for doping rare-earth ions as a luminescent material. Yi et al. have conducted

[13]

BiOCl using first-principles method

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systematic calculation of the electronic structure and optical properties of Eu-doped , and revealed that BiOCl: Eu3+ exhibits

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promising good luminescence properties. Armita et al. reported the photoluminescence of

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nanoflakes-like Eu: BiOX (X=Cl, Br, I) and suggested that both BiOCl and BiOBr are

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good host matrices for RE3+ ions to exhibit good luminescence properties as well as excellent photocatalytic properties under sunlight irradiation. The result shows that

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BiOX: Eu3+ matrices, bearing band gap energies of 3.21, 2.55 and 1.55 eV for X = Cl, Br, and I, respectively, exhibit 100 % photo-catalytic efficiency under visible light irradiation via RhB degradation

[14]

. Li et al. synthesized BiOCl: Eu3+ through solid-state method

possessing a broad excitation band in the near-ultraviolet region, the result showed that the phosphors exhibit great potential as red-emitting phosphors for white LED owing to excellent broadband NUV excitation ability and special far-red emission property [15]. However, the solid-state method in the previous part inevitably results in highenergy consumption, particle agglomeration, irregular and uncontrollable process, what’s more, the subsequent grinding will introduce surface defects which degrade luminescence 5 Page 5 of 45

efficiencies greatly [16]. To the best of our knowledge, few published articles are available to explore the luminescence properties of BiOCl: RE3+ (RE3+ = Eu3+, Sm3+) with threedimensional flower-like microarchitectures. Among various synthesis methods, the

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hydro/solvothermal process has been proved to be one of the most effective and convenient synthesis techniques in obtaining various hierarchical architectures, and this

[17]

. In this study, we present the

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effectively improve the optical properties of phosphors

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method can prevent particle agglomeration owing to its mild reaction conditions, thus

synthesis of Eu3+ and Sm3+-activated BiOCl with flower-like structure through facile

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solvothermal method, and the possible formation mechanism has been proposed based on the experimental results for the first time. The luminescence properties of Eu 3+ and Sm3+

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2. Experimental

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in the flowerlike BiOCl have been investigated in detail.

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2.1. Samples preparation

All chemical regents used in this experiment were of analytical grade and used

without further purification. Bi1-xEuxOCl and Bi1-ySmyOCl (x, y = 0.01, 0.03, 0.05, 0.07 and 0.09) were prepared by an ethylene glycol (EG)-assisted solvothermal method. Stoichiometric amounts of Eu(NO3)3, Sm(NO3 )3, Bi(NO3)3 • 5H2O and KCl were completely dissolved in 70 ml EG and stirred continuously to disperse all reagents homogeneously for several hours at room temperature. The reaction mixture was transferred into a 100 ml Teflon-lined stainless autoclave, and the autoclave was then placed in an oven with a temperature of 180 °C for 12 h. Afterward, the autoclave was 6 Page 6 of 45

allowed to cool to room temperature naturally. The resulting precipitate was collected and washed for several times with deionized water and ethanol. The obtained powders

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were dried in an oven (80 °C) for further characterization. 2.2. Characterizations

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X-ray powder diffraction (XRD) data were collected at ambient temperature with a

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D/Max 2550 diffractometer (Rigaku, Japan) at a scanning rate of 8° min-1 in the 2θ range from 10 to 70° with Cu Kα1 radiation, λ = 1.54056 Å, at 40 kV and 50 mA. The unit cell

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parameters of Eu and Sm doped BiOCl were determined from the X-ray powder diffraction patterns using the indexing program Fullprof [18]. The morphology was

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determined by field emission scanning electron (FE-SEM, S4800 Hitachi, Japan) microscopy. SEM–EDS elemental mapping was carried out using an EDAX instrument

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attached to a field-emission scanning electron (FEI Nova NanoSEM 450, Holland)

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microscope. The UV excitation and emission spectra of samples were recorded at room

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temperature using a fluorescence spectrophotometer (F-4600 Hitachi, Japan). The scan speed was fixed at 240 nm/min, the voltage was 700 V and the slits were fixed at 2.5 nm.

3. Results and discussions

3.1 Phase formation of BiOCl Powder X-ray diffraction patterns demonstrate that the Eu 3+ and Sm3+ doped

samples can be readily indexed to a pure tetragonal phase (space group: P4/nmm (No. 129)). All diffraction peaks are in good agreement with the values in JCPDS card No. 060249 (BiOCl). The incorporation of Eu3+ and Sm3+ into the host material can be 7 Page 7 of 45

evidenced by the estimation of unit cell parameters of the samples. Fig.1 shows the relationship between the refined unit cell volume of Bi1-xEuxOCl and Bi1-ySmyOCl with different concentrations of Eu3+ and Sm3+. It is found that the unit cell volumes nearly

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linearly decrease by about 0.18 and 0.15 % with increasing concentration of Eu3+ and Sm3+ in BiOCl, respectively. The ionic radii of Eu3+ (r = 1.066 Å, CN = 8) and Sm3+ (r =

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1.079 Å, CN = 8) is smaller than that of Bi3+ (r = 1.17 Å, CN = 8) [19]. As a consequence,

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the shrinkage of the unit cell volume is clearly observed as expected when Bi3+ are

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substituted by Eu3+ [20] and Sm3+.

Fig. 1 Eu3+ and Sm3+ concentration dependence of unit cell volume for Bi1-xEuxOCl and Bi1-ySmyOCl.

3.2 Morphology and possible formation mechanism for BiOCl with flower-like

globular microarchitectures Morphologies and particle size of the prepared BiOCl: Eu 3+ samples are shown in Fig. 2. An overview of the SEM images shows that all of the prepared samples have monodispersed flower-like globular microarchitectures with an average diameter about 5 8 Page 8 of 45

μm. These flower-like globular microarchitectures are composed of radically grown nanopetals with the length and thickness of 300 and 30 nm, respectively, as shown in the insets of Fig. 2(a-e), which are fabricated together to form opening microporous structure.

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The morphologies and particle sizes of these samples do not change much with increasing Eu3+ concentration, which means that various Eu3+ concentration exhibits little

The

surface

of

the

nanopetals

grown

on

the

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microarchitectures.

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influence on the particle size and morphology of the flower-like globular

microarchitectures was induced by both the Ostwald ripening process

globular [21]

and

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recrystallization process, which will be discussed in detail below.

Fig. 2 SEM images of Bi1-xEuxOCl samples: (a) x=0.01, (b) x= 0.03, (c) x=0.05, (d) x= 0.07, (e) x= 0.09, and the average particle size distribution with different Eu3+ concentrations.

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As shown in Fig. 3, Sm3+ doped BiOCl exhibits the similar morphology and particle size with BiOCl: Eu3+ because it was synthesized under the same conditions. In addition, the flower-like globular microarchitectures are composed of radically grown nanopetals

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with the length of about 500 nm and the thickness of 30 nm (insets of Fig.3 (a-e)). Herein, based on the above results, the possible formation mechanism of flower-like

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BiOCl: Eu3+ with globular microarchitectures is proposed. During the typical synthesis of

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the flower-like BiOCl: Eu3+, intermediate samples were taken at different time intervals and then characterized by SEM to reveal the formation process. As shown in Fig. 4, at the

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beginning, some numerous tiny nanoparticles and few nanopetals were formed in the presence of urea in EG solution (Fig. 4a). As the time was extended to 0.5 h, these

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nanoparticles started to form big sphere, a typical Ostwald ripening transformation from

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dispersed nanoparticle to microsphere was observed (Fig. 4b). Some nanopetals with

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small size are formed on the spherical surface leading to the recrystallization process. As the time was extended to 6 h (Fig. 4c), during the crystal growth process, EG molecules

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could selectively be adsorbed on the surface of the nanopetals by forming hydrogen bonds with the exposed oxygen atom on facet of BiOCl. When the time was prolonged to 12 h, the nanopetals continued to grow and totally formed the flower-like structure (Fig. 4d). The change of unit cell volume of samples with reaction time increasing was indexed (Fig. 4e). The

schematic

illustration

of

the

growth

process

of

BiOCl

globular

microarchitectures was shown in Fig. 4. During this process, small primary building blocks (i.e. nanopetals) automatically assemble through an energetically-favored oriented attachment mechanism, because the formation of larger crystal can reduce the surface 10 Page 10 of 45

energy of small building blocks

[22]

. This formation mechanism has also been

demonstrated for the crystal growth of other materials, such as Y 2O3 [25]

[23]

, ZnO

[24]

, MnO

. The unit cell volumes of samples with increasing reaction time have been indexed

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(Fig. 4e). There is a small decrease (about 0.2 %) for the unit cell volume with increasing reaction time from 0-12 h. This observation is in good agreement with the fact that the

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larger particle size and better crystallinity with an increase of reaction time.

Fig. 3 SEM images of Bi1-ySmyOCl samples: (a) y=0.01, (b) y= 0.03, (c) y=0.05, (d) y= 0.07, (e) y= 0.09, and the average particle size distribution with different Sm3+ concentrations.

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Fig. 4 SEM images (a-d) and unit cell volumes (e) of flower-like BiOCl: Eu3+ globular microarchitectures

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at different time intervals: (a) 0h, (b) 0.5 h, (c) 6 h, (d) 12 h, and the schematic illustration of the growth

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process of BiOCl globular microarchitectures.

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3.4 Photoluminescence of Eu3+ and Sm3+ activated BiOCl

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Fig. 5 Emission (a) and excitation (b) spectra of Bi1-xEuxOCl (x = 0.01, 0.03, 0.05, 0.07 and 0.09).

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Fig. 5 shows the excitation and emission spectra of Bi1-xEuxOCl (x = 0.01, 0.03, 0.05, 0.07 and 0.09). Eu3+ activated BiOCl emits red light under 327 nm excitation. The typical emission spectra are presented in Fig. 5 (a). The emission lines of Eu 3+ ion at about 584, 592, 616 (621, 628), 652 (669) and 698 nm corresponding to the transitions from the excited 5D0 level to the 7FJ (J = 0, 1, 2, 3, 4) levels of the 4f6 configuration are observed. In addition, the emission peaks at 536 and 556 nm related to the 5D1 → 7F1 and 5D1 → 7F2 transitions are observed. It is interesting to note that these emission peaks of Eu3+ are dominated by the peaks at 698 nm (5D0 → 7F4 transition), which is abnormal from most of the Eu3+-activated phosphors

[12]

. Moreover, the emission intensity of the electric

dipole transition (5D0 → 7F2) is considerably stronger than that of the magnetic dipole 13 Page 13 of 45

transition (5D0 →7F1), indicating that the Eu3+ ions occupy the sites without inversion symmetry, which is in good agreement with the site occupation of Eu 3+ on Bi3+ sites in

means that there is no concentration quenching till 7 mol%.

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BiOCl [26]. The emission intensity increases with increasing Eu 3+ concentration, which

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As shown in Fig. 5 (b), there are two principle excitation bands in the excitation spectra of Bi1-xEuxOCl monitored the 5D0 → 7F2 transition, when measured at room

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temperature: one is located in the wavelength range of 225 - 350 nm, and the other one is

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in the visible spectral range of 350 - 450 nm. According to the calculation result, the predicted charge transfer energy of Eu3+ in BiOCl host lattice is 4.3 eV, i.e. locates at

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about 290 nm in the deep-ultraviolet range [15]. The first broad band with the maximum at about 308 nm (i.e. 4.05 eV) can be attributed to the charge transfer band (CTB) of Eu3+, [14]

.

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which arises from the transition of 2p electrons of O2- to the 4f orbitals of Eu2+ ions

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The energy of CTB of Eu3+ in BiOCl microarchitectures is lower compared to that of

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calculation [13]. As well known, the CTB is strongly related to the covalency between O2and Eu3+. A decrease in energy for electron transfer from O2- to Eu3+ indicates an increase in covalency, i.e. a decrease in iconicity between O2- and Eu3+ [27]. The shoulder at 275 nm originates from the host absorption characteristic of the 1S0 → 3P1 transition of Bi3+, which is similar to the results observed in Bi3+/Eu3+-co-doped Ca10(PO4)6F2 [28]. The weak peaks in the wavelength range of 350 - 500 nm are related to the intra-configuration 4f 4f transitions of Eu3+ ion. These peaks at 394 and 465 nm corresponding to 7F0 → 5L6 and 7

F0 → 5D2 transitions of Eu3+ ions in the host lattice were observed. However, there is no

sign for the existence of Eu2+, due to the absence of Eu2+ 5d - 4f broad band transitions in the emission and excitation spectra. Eu3+-doped BiOCl shows great potential for 14 Page 14 of 45

application as an alternative red-emitting LED conversion phosphor due to its efficient

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red emission and strong excitation bands in the wavelength range of 275 - 425 nm.

Fig. 6 Emission (a) and excitation (b) spectra of Bi1-ySmyOCl (y = 0.01, 0.03, 0.05, 0.07 and 0.09).

Trivalent samarium with 4f5 electron configuration has complicated energy levels

and various possible transitions between f levels. As a consequence, the f - f transitions are highly selective and of sharp line emissions. Divalent samarium has the 4f6 electron configuration, which under irradiation with UV and visible light can be excited into the 4f55d1 levels. As shown in Fig. 6(a), Sm3+-activated BiOCl exhibits reddish orange 15 Page 15 of 45

emission under 407 nm excitation. The emission spectra are composed of 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2) transitions of Sm3+, which are located at (561) 566 nm (4G5/2 → 6H5/2), (598) 607 nm (4G5/2 → 6H7/2) and (648) 658 nm (4G5/2 → 6H9/2), respectively. Among

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them, the orange emission at 598 nm ( 4G5/2 → 6H7/2) exhibits the strongest emission intensity. As shown in the Fig. 6(a), the integrated emission intensity decreases as the

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Sm3+ concentration exceeds 5 mol% as a consequence of concentration quenching. In

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addition, there is no clear-cut evidence for the presence of Sm2+, since no f - f transitions

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of Sm2+ could be identified.

For Fig. 6(b), the excitation spectra of Sm3+-activated BiOCl were shown by

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monitoring the emission band at 598 nm. The main peaks of UV excitation spectra in the region of 350 - 480 nm are due to the excitation from the ground level 6H5/2 to higher

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energy levels (4F9/2, 4D5/2, 6P7/2, 4K11/2, 6P5/2, 4M19/2, 4G9/2, 4I15/2, 4F5/2, 4I13/2) of Sm3+.

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Normally, the typical Sm3+ activated oxide-based phosphors show the charge transfer

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band of Sm3+ (i.e. CTB of O2--Sm3+) in the UV region. Herein, the broad band at about 246 nm (i.e. 5.05 eV) can be attributed to the CTB, resulting from an electron transfer from O2- (2p6) orbital to the empty state of 4f6 of Sm2+ ion. In addition, the CTB of Sm3+ in most of the hosts appears to be about 1.16 eV higher in energy than that of Eu 3+

[29]

.

For Eu3+-activated BiOCl, the CTB of Eu3+ is about 3.99 eV (311 nm) aforementioned, therefore, the CTB energy of O2--Sm3+ should be at about 5.15 eV (241 nm), which is in good agreement with the experimental result. To examine the distribution of Eu and Sm ions in three-dimensional flower-like BiOCl globular microarchitectures, which exhibits a strong influence on the luminescence properties, elemental mapping by SEM energydispersive X-ray spectrometry (SEM/EDS) was carried out for Eu- and Sm-doped 16 Page 16 of 45

samples, as shown in Fig. 7. It can be seen clearly that Bi, O, Cl, as well as Eu and Sm ions were homogeneously dispersed in the three-dimensional flower-like BiOCl globular

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microarchitecture, leading to the efficient luminescence properties.

Fig. 7 SEM-EDS elemental mapping for Bi, Cl, O and Eu (Sm) elements in Eu (a-e) and Sm-doped (f-j)

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4. Conclusion

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BiOCl with three-dimensional flower-like globular microarchitecture.

In summary, uniformly dispersed Eu3+ and Sm3+ -activated BiOCl with flower-like

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structure have been successfully prepared through a facile solvothermal method using urea as the structure-directing reagent. The unit cell volumes decrease linearly by about 0.18 and 0.15 % with increasing Eu3+ and Sm3+ concentration up to 9 mol%, respectively. All of the prepared micro-particles show flower-like globular microarchitectures with an average diameter about 3 to 5 μm. In addition, Eu and Sm ions were homogeneously dispersed in the three-dimensional globular microarchitectures. Possible formation mechanism for the flower-like microstructure has been proposed based on the timedependent experiment. Photoluminescence results show that Eu3+ and Sm3+ -activated BiOCl with flower-like structure exhibit a strong red emission corresponding to the 17 Page 17 of 45

typical f - f transitions of Eu3+ and Sm3+, respectively. The charge transfer bands of Eu 3+

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and Sm3+ located at 311 and 241 nm have been observed.

Acknowledgments

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This research was supported by Basic Science Research Program through the

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National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.

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2014R1A1A2058415) and (No.2015R1D1A1A1058991).

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[22] R.Q. Song, H. Cölfen, Additive controlled crystallization, CrystEngComm. 13 (2011) 1249-1276.

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[23] X.-M. Zhang, M.-L. Huang, Z.-J. Zhang, B.-Q. Liu, J.-T. Zhao, Daisy-like Y(OH)3:Eu/Y2O3:Eu microstructure: formation and luminescence properties, Mater. Lett.

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68 (2012) 269-272.

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[24] C. Pacholski, A. Kornowski, H. Weller, Self-assembly of ZnO: from nanodots to

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nanorods, Angew. Chem. Int. Ed. 41 (2002) 1188-1191.

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[25] D. Zitoun, N. Pinna, N. Frolet, C. Belin, Single crystal manganese oxide multipods by oriented attachment, J. Am. Chem. Soc. 127 (2005) 15034-15035. [26] X. Li, Y. Zhang, D. Geng, J. Lian, G. Zhang, Z. Hou, J. Lin, CaGdAlO 4:Tb3+/Eu3+ as promising phosphors for full-color field emission displays, J. Mater. Chem. C. 2 (2014) 9924-9933.

[27] N. Zhang, W.B. Bu, Y.P. Xu, D.Y. Jiang, J.L. Shi, Self-assembled flowerlike europium-doped lanthanide molybdate microarchitectures and their photoluminescence properties, J. Phys. Chem. C. 111 (2007) 5014-5019.

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[28] H. Zhu, Z. Xia, H. Liu, R. Mi, Z. Hui, Luminescence properties and energy transfer of Bi3+/Eu3+-codoped Ca10(PO4)6F2 phosphors, Mater. Res. Bull. 48 (2013) 3513-3517.

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[29] P. Dorenbos, Systematic behaviour in trivalent lanthanide charge transfer energies, J.

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Phys. Condens. Mat. 15 (2003) 8417-8434.

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Elsevier Editorial System(tm) for Sensors & Actuators: B. Chemical Manuscript Draft Manuscript Number: SNB-D-15-02609R1 Title: Reflectance-Based Detection of Oxidizers in Ambient Air

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Article Type: Short Communication Section/Category: General section

Corresponding Author: Dr. Brandy J. Johnson, PhD

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Keywords: peroxide; reflectance; portable sensor; vapor detection; paperbased

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Corresponding Author's Institution: Naval Research Laboratory First Author: Brandy J. Johnson, PhD

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Manuscript Region of Origin: USA

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Order of Authors: Brandy J. Johnson, PhD; Ray Liu; Robert C Neblett II; Anthony P Malanoski, PhD; Miao Xu, PhD; Jeffrey S Erickson, PhD; Ling Zang, PhD; David A Stenger, PhD; Martin H Moore, MS

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Response to Reviewers

December 2, 2015

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Dr. R. Narayanaswamy The University of Manchester Editor, Sensors and Actuators B: Chemical

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Dear Sir:

We have revised our manuscript entitled “Reflectance-Based Detection of Oxidizers in Ambient Air”

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according to the reviewer suggestions and are resubmitting it for consideration for publication in Sensors and Actuators B as a short communication. This manuscript represents new and unpublished results

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focused on the use of paper-supported, titanium-based indicators for hydrogen peroxide vapor sensing in combination with a prototype reflectance sensor. Paper-supported porphyrin indicators are provided for comparison. The advantages of each type of indicator are discussed. This work is not under consideration

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for publication by any other source. We have no conflicts of interest to declare. Each of the authors is familiar with the contents of the article and has approved its publication. This effort is funded by the US Office of Naval Research through the Naval Research Laboratory. The sponsors did not contribute to

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experimental design, data interpretation, or any other component of this manuscript.

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Below, you will find our point-by-point response to the reviewers. Thank you in advance for your time and attention to this matter.

Best regards,

Dr. Brandy J. Johnson Research Chemist Center for Bio/Molecular Science and Engineering Naval Research Laboratory Washington, DC 20375 [email protected] Ph (202) 404-6100

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Reviewer #2: This paper presents an interesting method for the determination of oxidizers in air. Using previously reported titanium indicators and a prototype reflectance sensor, the authors have developed a system for the detection of hydrogen peroxide that might work in real applications. Therefore, this work may be of interest for the readers of Sensors and Actuators: B. However, I have some comments that the authors must consider:

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- The authors state that "The experiments of the current study were not conducted in a sealed headspace nor did they utilize a large target excess; concentrations of target were expected to change over time." The exposure to changing concentrations is certainly more interesting than a constant environment for the characterization of sensors. However, in this case the concentrations are unknown and uncontrolled. A quantification of the concentrations, or at least an approximation, would be helpful to understand the behavior of the sensor. Thank you for your suggestion. We have included an estimate of the maximum initial concentration based on the size of the headspace at the time the bottle was opened. (Experimental Section, paragraph 3)

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- Regarding the lack of reversibility, have the authors tried to recover the titanyl indicators by any means? An obvious yet perhaps unpractical way would involve the use of reducing agents. The lack of reversibility is the main flaw of these titanium indicators, while those based on porphyrins lack of specificity because of the similar effects caused by oxidation and protonation. By providing some possibilities to achieve sensor recovery, the authors would enhance the possibilities of this system. The indicators are drop-cast onto the paper support, so they cannot be treated with a liquid reducing agent; treatment with liquid will result in migration of the indicator and/or its removal from the paper support. The sensing reaction product (titanium peroxide) can be regenerated by hydrolysis in basic water (pH>7); the precipitate titanium hydroxide thus formed can be dissolved in oxalic acid to reproduce the titanium-oxo oxalate salt. It should be noted that this indicator was developed to be disposable. Considering the low cost and non-toxicity of the titanium oxalate material, the cost of recovery would be much higher than the material itself. Additional comments to this effect have been added. (Results, paragraph 3) Reviewer #3: This study used two types of paper supported materials with a prototype, reflectance-based detector for indication of hydrogen peroxide vapor under ambient laboratory conditions. However, the novelty and mechanism are not clear. The language is also hard to understand. There should be a scheme to demonstrate the principle underlying this study. We have included a scheme for reaction of the titanyl indicator with peroxide. (Introduction, paragraph 2) This should help to clarify the mechanism involved. We have made modifications throughout the text in an attempt to address concerns about the difficulty of the language.

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*Manuscript - revised, clean Click here to view linked References

Reflectance-Based Detection of Oxidizers in Ambient Air Brandy J. Johnsona,*, Ray Liub, Robert C. Neblett IIc, Anthony P. Malanoskia, Miao Xud, Jeffrey S. Ericksona, Ling Zangd, David A. Stengera, Martin H. Moorea a

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Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375, USA b

Thomas Jefferson High School for Science & Technology, Alexandria, VA 22312, USA; at NRLa Summer 2015 through SEAP internship Biology Department, Howard University, Washington, DC 20059, USA; at NRLa Summer 2015 through ONR internship

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Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84108, USA

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*corresponding author. Tel. 2024046100; email: [email protected]

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ABSTRACT.

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This study used two types of paper supported materials with a prototype, reflectance-based detector for indication of hydrogen peroxide vapor under ambient laboratory conditions. Titanyl based indicators provide detection through reaction of the indicator resulting in a dosimeter type sensor, while porphyrin based indicators provide a reversible interaction more suitable to continuous monitoring applications. These indicators provide the basis for discussion of characteristics important to design of a sensor system including the application environment and duration, desired reporting frequency, and target specificity. KEYWORDS. peroxide, reflectance, portable sensor, vapor detection, paper-based

INTRODUCTION.

Peroxide-based homemade explosives have been identified as a threat by the US Department of Homeland Security and the US Department of Defense. Numerous online articles warn first responders of the threat these materials present, the ease of their synthesis, and their inherently unstable nature. Preparation can be as simple as mixing household chemicals, such as acetone, acid, and peroxide, making the materials favored for incorporation into improvised explosive devices (IEDs). Triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), tetramethylene diperoxide dicarbamide (TMDD) and related cyclic organic peroxides are examples of specific compounds, but peroxide-based explosives can be used in either liquid or solid forms. Liquids, for example, have been used in terrorist incidents, including the 2005 attacks on transit systems in London and the foiled airline attacks of 2009. The threat posed by these compounds has resulted in development of a wide range of detection approaches for single compounds (i.e., TATP) utilizing techniques from mass

Page 26 of 45

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spectrometry through portable electrochemical approaches.[1-3] A more general method could target the hydrogen peroxide present in the liquid explosive materials and often found as an impurity and/or degradation product remaining in the solids utilized. While colorimetric detection methods for hydrogen peroxide in solution are widely available,[4-12] those available for gas phase detection are more limited.[3] A paper-based material has been reported for detection of hydrogen peroxide vapor.[13] The material relies on the interaction of peroxide with ammonium titanyl oxalate resulting in a change from white to yellow (Scheme 1). Titanium based indicators have been used by a number of groups for detection of hydrogen peroxide.[712] The recent report, however, utilizes a paper support, providing a large surface area for interaction of target with the indicator as well as an open pore network for ease of diffusion throughout the material.[13] Selectivity for hydrogen peroxide was demonstrated with no response to ethanol, methanol, acetone, tetrahydrofuran, hexane, toluene, ethyl acetate, or chloroform. In the current study, the paper supported titanyl indicator is used with a prototype chemical sensor. The sensor hardware has previously been described for use with paper supported porphyrin indicators for the detection of alcohol vapor.[14] It relies on an array of commercially available color sensors and provides data output consisting of white, red, green, and blue color values. Data is collected in five second intervals, allowing for rapid determination of target presence. While the previous work characterized the titanium indicators under highly controlled conditions, the prototype sensor of this study provided the opportunity to look at indicator performance in the ambient environment with conditions changing in real-time.

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For synthesis of peroxide specific indicators, ammonium titanyl oxalate monohydrate was purchased from Sigma Aldrich and used as received. The paper support materials were purchased from Whatman (Cat No. 1001 150). Loading of the paper support was accomplished using 100 μL of 20 M ammonium titanyl oxalate monohydrate in water which was drop-cast onto a 2.5 x 2.5 cm swatch.[13] This was followed by drying under vacuum at room temperature for 1 h. Cobalt (CoDIX), copper (CuDIX), and nickel (NiDIX) variants of Deuteroporphyrin IX bis ethyleneglycol (CAS 6239456-72-5) were prepared by reflux as previously reported.[14, 15] Paper supported porphyrin indicators were prepared using a dip and dry technique. For a 5 x 33 cm swatch, 0.4 mM porphyrin in water (total volume 6 mL) was used. The paper support (WypAll X60) was pulled through this solution and allowed to dry slightly before being pulled through the solution again. This was repeated until all porphyrin solution had been deposited (typically three cycles). Samples were then dried at 100°C before storing in the dark in sealed plastic bags. This is a modification of the procedure described previously for preparation of the indicator materials.[14] The prototype reflectance instrument utilized low cost, commercially available color sensing breakout boards from Parallax, Inc. (model TCS3200-DB, Rocklin, CA), providing a color lightto-frequency integrated circuit from AMS (model TCS3200, Plano, TX), a pair of white LEDs, and an adjustable lens. The device was previously described in detail.[14] Briefly, six of the breakout boards were used with a customized multiplex platform in which the boards were mounted using in-house developed holders made from chemically resistant Delrin plastic

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(McMaster-Carr, Princeton, NJ). The indicator support provides a lip on the bottom that can sit on top of a Petri dish or bottle (Figure 1). The device output consists of a stream of digital pulses proportional to the intensity of the color being measured. A custom printed circuit board (PCB) interfaces with and controls the six sensors. The PCB uses an Atmel ATMega microcontroller (Atmel Corporation, San Jose, CA) to regulate the timing of events, count pulses, and report the results to a computer. Communications between the instrument and the computer are via USB; power is supplied through a dc barrel jack. A LabWindows developed software-based graphical user interface (GUI) communicates with the PCB firmware through simple ASCII commands. Target exposure was completed using 200 mL Nalgene bottles containing a solution volume of 30 mL. Target solutions consisted of deionized water; dilutions of 30% hydrogen peroxide; dilutions of sulfuric, hydrochloric, and nitric acid; and solvents such as ethanol and acetone. H2O2 solutions of 3, 1.2, 0.3, 0.15, and 0.06% in water provided maximum initial headspace concentrations of 8.45, 3.38, 0.84, 0.42, and 0.17 ppm, respectively. Solutions were prepared and capped for 2 to 4 h to allow for headspace equilibration prior to exposure of indicators. Data was collected for a minimum of five minutes prior to target exposure to establish a baseline for the indicators. For this measurement, bottles containing 30 mL of water were used as the control solutions. Exposure was initiated by exchanging water containing bottles for those containing target. An alternative approach accomplished target exposure by placing the indicator supports over empty Petri dishes for pre- and post-exposure measurements. The exposure measurements were completed by placing the holder over a Petri dish (60 mm; total volume 57 mL) containing 1 mL of warmed target solution (10 min in oven at 60°C).

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Results and Discussion. The titanyl indicators were evaluated using the prototype reflectance sensor with target (30 mL) in 200 mL Nalgene bottles. This experiment is significantly different from those described in the original report.[13] Initial characterization utilized a sealed 9 L headspace over 1 L of target solution with a fan generating impacting air flow. Under these conditions, the target content at even the lowest utilized concentrations (0.1 ppm vapor) would not change over the course of the measurement. The experiments of the current study were not conducted in a sealed headspace nor did they utilize a large target excess; concentrations of target were expected to change over time. Ambient temperatures were between 24 and 27°C with relative humidity between 43 and 55%. Though the prototype device reports red, green, and blue (RGB) color values (Figure 2, Panel A), changes observed for the titanyl indicators were much more dramatic for the blue channel than for the red or green. The data has been normalized using the average value from the pre-exposure measurement to account for sensor to sensor variation in the data. In Figure 2 (Panel B), we report the changes in the blue channel over time following exposure of indicators to various concentrations of hydrogen peroxide. The rates of change in reflectance for the titanyl indicators (blue and green values only) were found to be concentration dependent with saturation of the indicator occurring at ~9.5 h for the 3% target solution and ~15.5 h for the 1.2% target solution. Measurements were continued to 66.5 h; none of the lower target concentrations resulted in indicator saturation. (Additional results provided in the Supplementary Material) Previous work with the prototype sensor utilized porphyrin indicators and focused on chemosorptive interactions,[14, 16] often of a reversible nature. The titanyl compounds utilize a reactive interaction (non-reversible) with the peroxide vapor. Representative porphyrin indicators were evaluated for comparison to the titanyl materials (Figure 3). The CoDIX indicator showed significant changes in reflectance upon exposure to hydrogen peroxide while

Page 28 of 45

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changes in CuDIX reflectance were smaller and NiDIX did not respond. The changes in CoDIX and CuDIX were found to be reversible when target was removed from the indicator environment. Porphyrin indicators were also found to respond to the presence of sulfuric, nitric, and hydrochloric acid (additional results provided in the Supplementary Material). As previously reported, these DIX variants are also sensitive to alcohol vapors.[14] The reflectance of the titanyl indicators does not change upon exposure to acids or alcohols; they provide unique indication of the presence of peroxide vapor. This specificity may offer a significant advantage depending on the application, especially if peroxide vapor is a primary or high value target. There are other aspects that should be considered when designing a sensor system. The differences between the porphyrin and titanyl indicators serve to illustrate some of these points. The titanyl indicators are dosimetry type reporters; that is, the reported signal is an integration of exposure concentrations and durations. These indicators will reach a saturation point after which they are no longer useful for detection of target. The porphyrin indicators, on the other hand, respond to increasing and decreasing concentrations with reversible changes in reflectance. This provides the potential for continuous or long term applications but makes them unsuitable for reporting on total exposure. The titanyl indicators could be regenerated through hydrolysis in basic water followed by reaction with oxalic acid; however, treatment of the paper supported indicator with liquids will result in migration. Regeneration would likely also be more expensive than replacement of the indicator. A further consideration in characterization of indicator materials for sensor applications is quantification of target concentrations. The reactive nature of the titanyl indicators was originally described as providing the potential for determination of concentrations of peroxide vapor on the basis of rates of change in indicator color.[13] This description should be qualified to be specific for situations under which the sampled space is stagnant and of sufficient volume to prevent reactant depletion. The rate equations developed under the original work cannot be applied to the experimental conditions utilized in the current study. Time is also an important consideration for the proposed type of analysis. These are not necessarily a realizable set of conditions. Dramatic fluctuations in target concentration would be expected for point sensors in an environmental sensing scenario.[17, 18] Acknowledgements. This research was sponsored by the U.S. Naval Research Laboratory (WU# 69-6594). R. Neblett was supported through an Office of Naval Research (ONR) sponsored summer research internship. Participation of R. Liu was through the US Navy Science and Engineering Apprenticeship Program (SEAP). The views expressed here are those of the authors and do not represent those of the U.S. Navy, the U.S. Department of Defense, or the U.S. Government. Scheme 1. Modification of titanyl indicator by peroxide resulting in yellow color. Figure 1. Photograph of the prototype sensor system with fresh (A) and exposed (B) titanyl indicators and (C) the CoDIX indicator. Figure 2. Response of paper supported titanyl indicators to hydrogen peroxide. (A) Shown here are the as reported red, green, and blue color values (dark to light) for an indicator swatch before and during exposure to the vapor evolving from a solution of 3% H2O2 in water (dashed line marks beginning of exposure period). (B) The normalized blue color values are reported for

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indicator swatches before and during exposure to vapor evolving from water (black) and 3, 1.2, 0.3, 0.15, and 0.06% (dark to light) solutions of H2O2 in water.

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Figure 3. Response of paper porphyrin indicator to hydrogen peroxide. (A) Shown here are the normalized red, green, and blue color values (dark to light) for a CoDIX indicator swatch before and during exposure to the vapor evolving from a solution of 3% H2O2 in water (dashed line marks beginning of exposure period). (B) The normalized blue color values are reported for CoDIX indicator swatches before and during exposure to vapor evolving from water (black), 3, 1.2, 0.3, and 0.15% (dark to light) solutions of H2O2 in water.

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References. [1] R.M. Burks, D.S. Hage, Current trends in the detection of peroxide-based explosives, Analytical and Bioanalytical Chemistry, 395(2009) 301-13. [2] M.E. Germain, M.J. Knapp, Optical explosives detection: from color changes to fluorescence turn-on, Chemical Society Reviews, 38(2009) 2543-55. [3] T. Krawczyk, S. Baj, Review: Advances in the determination of peroxides by optical and spectroscopic methods, Analytical Letters, 47(2014) 2129-47. [4] R. Bailey, D.F. Boltz, Differentical spectrophotometric determination of hydrogen peroxide using 1,10-phenanthroline and bathophenanthroline, Analytical Chemistry, 31(1959) 117-9. [5] C. Matsubara, K. Kudo, T. Kawashita, K. Takamura, Spectrophotometric determination of hydrogen-peroxide with titanium 2((5-bromopyridyl)azo)-5-(N-propyl-Nsulfopropylamino)phenol reagent and its application to the determination of serum glucose using glucose-oxidase, Analytical Chemistry, 57(1985) 1107-9. [6] K. Takamura, C. Matsubara, Versatility of the Titanium(IV) - Porphyrin reagent for determining hydrogen peroxide, Bulletin of the Chemical Society of Japan, 76(2003) 1873-88. [7] G.M. Eisenberg, Colorimetric determination of hydrogen peroxide, Industrial and Engineering Chemistry-Analytical Edition, 15(1943) 327-8. [8] B.D. Patterson, E.A. Macrae, I.B. Ferguson, Estimation of hydrogen-peroxide in plantextracts using titanium (IV), Analytical Biochemistry, 139(1984) 487-92. [9] C. Matsubara, N. Kawamoto, K. Takamura, Oxo 5,10,15,20-tetra(4-pyridyl)porphyrinato titanium(IV) - An ultra-high sensitivity spectrophotometric reagent for hydrogen-peroxide, Analyst, 117(1992) 1781-4. [10] F.I. Bohrer, C.N. Colesniuc, J. Park, I.K. Schuller, A.C. Kummel, W.C. Trogler, Selective detection of vapor phase hydrogen peroxide with phthalocyanine chemiresistors, Journal of the American Chemical Society, 130(2008) 3712-3. [11] R.M. Sellers, Spectrophotometric determination of hydrogen peroxide using potassium Titanium (IV) oxalate, Analyst, 105(1980) 950-4. [12] J. Muhlebach, K. Muller, G. Schwarzenbach, The peroxo complexes of titanium, Inorganic Chemistry, 9(1970) 2381-90. [13] M. Xu, B.R. Bunes, L. Zang, Paper-Based Vapor Detection of Hydrogen Peroxide: Colorimetric Sensing with Tunable Interface, Acs Applied Materials & Interfaces, 3(2011) 6427. [14] B.J. Johnson, J.S. Erickson, J. Kim, A.P. Malanoski, I.A. Leska, S.M. Monk, et al., Miniaturized reflectance devices for chemical sensing, Measurement Science & Technology, 25(2014) 095101 (10pp).

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[15] B.J. Johnson, N.E. Anderson, P.T. Charles, A.P. Malanoski, B.J. Melde, M. Nasir, et al., Porphyrin-embedded silicate materials for detection of hydrocarbon solvents, Sensors, 11(2011) 886-904. [16] L. Feng, C.J. Musto, J.W. Kemling, S.H. Lim, K.S. Suslick, A colorimetric sensor array for identification of toxic gases below permissible exposure limits, Chem Comm, 46(2010) 2037-39. [17] P.R. Best, K.E. Lunney, C.A. Killip, Statistical elements of predicting the impact of a variety of odour sources, Water Sci Technol, 44(2001) 157-64. [18] K. Obenschain, J. Boris, G. Patnaik, Using CT-Analyst to Optimize Sensor Placement, Defense and Security, (2004) 14-20.

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*Manuscript - revised, marked Click here to view linked References

Reflectance-Based Detection of Oxidizers in Ambient Air Brandy J. Johnsona,*, Ray Liub, Robert C. Neblett IIc, Anthony P. Malanoskia, Miao Xud, Jeffrey S. Ericksona, Ling Zangd, David A. Stengera, Martin H. Moorea a

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Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC 20375, USA b

Thomas Jefferson High School for Science & Technology, Alexandria, VA 22312, USA; at NRLa Summer 2015 through SEAP internship Biology Department, Howard University, Washington, DC 20059, USA; at NRLa Summer 2015 through ONR internship

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Department of Materials Science and Engineering, University of Utah, Salt Lake City, UT 84108, USA

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*corresponding author. Tel. 2024046100; email: [email protected]

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ABSTRACT.

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This study used two types of paper supported materials with a prototype, reflectance-based detector for indication of hydrogen peroxide vapor under ambient laboratory conditions. Titanyl based indicators provide reaction based detection through reaction of the indicator resulting in a dosimeter type sensor, while porphyrin based indicators provide a reversible interaction more suitable to continuous monitoring applications. These indicators provide the basis for discussion of characteristics of importantce to design of a sensor system including the application environment and duration, desired reporting frequency, and target specificity. KEYWORDS. peroxide, reflectance, portable sensor, vapor detection, paper-based

INTRODUCTION.

Peroxide-based homemade explosives have been identified as a threat by the US Department of Homeland Security and the US Department of Defense. Numerous online articles warn first responders of the threat these materials present, the ease of their synthesis, and their inherently unstable nature. Preparation can be as simple as mixing household chemicals, such as acetone, acid, and peroxide, making the materials a favored ingredient for incorporation into improvised explosive devices (IEDs). Triacetone triperoxide (TATP), hexamethylene triperoxide diamine (HMTD), tetramethylene diperoxide dicarbamide (TMDD) and related cyclic organic peroxides are examples of specific compounds, but peroxide-based explosives can be used in either liquid or solid forms. Liquids, for example, have been used in terrorist incidents, such asincluding the 2005 attacks on transit systems in London and the foiled airline attacks of 2009.

Page 32 of 45

Experimental.

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The threat posed by these compounds has resulted in development of a wide range of detection approaches for single compounds (i.e., TATP) utilizing techniques from mass spectrometry through portable electrochemical approachetechniques.[1-3] A more general method could target the hydrogen peroxide which is present in the liquid explosive materials and is often found as an impurity and/or degradation product remaining in the solids utilized. While colorimetric detection methods for hydrogen peroxide in solution are widely available,[4-12] those available for gas phase detection are more limited.[3] A paper-based material has been reported for detection of hydrogen peroxide vapor.[13] The material relies on the interaction of peroxide with ammonium titanyl oxalate resulting in a change from white to yellow (Scheme 1). Titanium based indicators have been used by a number of groups for detection of hydrogen peroxide.[7-12] The recent report, however, utilizes a paper support, providing a large surface area for interaction of target with the indicator as well as an open pore network for ease of diffusion throughout the material.[13] Selectivity for hydrogen peroxide was demonstrated with no response to ethanol, methanol, acetone, tetrahydrofuran, hexane, toluene, ethyl acetate, or chloroform. In the current study, the paper supported peroxide titanyl indicator is used with a prototype chemical sensor. The sensor hardware has previously been described for use with paper supported porphyrin indicators for the detection of alcohol vapor.[14] It relies on an array of commercially available color sensors and provides data output consisting of white, red, green, and blue color values. Data is collected in five second intervals, allowing for rapid determination of target presence. While the previous work characterized the titanium indicators under highly controlled conditions, the prototype sensor of this study provided the opportunity to look at indicator performance in the ambient environment with conditions changing in real-time.

Ac ce p

For synthesis of peroxide specific indicators, ammonium titanyl oxalate monohydrate was purchased from Sigma Aldrich and used as received. The paper support materials were purchased from Whatman (Cat No. 1001 150). Loading of the paper support was accomplished using 100 μL of 20 M ammonium titanyl oxalate monohydrate in water which was drop-cast onto a 2.5 x 2.5 cm swatch.[13] This was followed by drying under vacuum at room temperature for 1 h. Cobalt (CoDIX), copper (CuDIX), and nickel (NiDIX) variants of Deuteroporphyrin IX bis ethyleneglycol (CAS 6239456-72-5) were prepared by reflux as previously reported.[14, 15] Paper supported porphyrin indicators were prepared using a dip and dry technique. For a 5 x 33 cm swatch, 0.4 mM porphyrin in water (total volume 6 mL) was used. The paper support (WypAll X60) was pulled through this solution and allowed to dry slightly before being pulled through the solution again. This was repeated until all porphyrin solution had been deposited (typically three cycles). Samples were then dried at 100°C before storing in the dark in sealed plastic bags. This is a modification of the procedure described previously for preparation of the indicator materials.[14] The prototype reflectance instrument utilized low cost, commercially available color sensing breakout boards from Parallax, Inc. (model TCS3200-DB, Rocklin, CA), providing a color lightto-frequency integrated circuit from AMS (model TCS3200, Plano, TX), a pair of white LEDs, and an adjustable lens. The device was previously described in detail.[14] Briefly, six of the

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breakout boards were used with a customized multiplex platform in which the boards were mounted using in-house developed holders made from chemically resistant Delrin plastic (McMaster-Carr, Princeton, NJ). The indicator support provides a lip on the bottom that can sit on top of a Petri dish or bottle (Figure 1). The device output consists of a stream of digital pulses proportional to the intensity of the color being measured. A custom printed circuit board (PCB) interfaces with and controls the six sensors. The PCB uses an Atmel ATMega microcontroller (Atmel Corporation, San Jose, CA) to regulate the timing of events, count pulses, and report the results to a computer. Communications between the instrument and the computer are via USB; power is supplied through a dc barrel jack. A LabWindows developed software-based graphical user interface (GUI) communicates with the PCB firmware through simple ASCII commands. Target exposure was completed using 200 mL Nalgene bottles containing a solution volume of 30 mL. Target solutions consisted of deionized water; dilutions of 30% hydrogen peroxide; dilutions of sulfuric, hydrochloric, and nitric acid; and solvents such as ethanol and acetone. H2O2 solutions of 3, 1.2, 0.3, 0.15, and 0.06% in water provided maximum initial headspace concentrations of 8.45, 3.38, 0.84, 0.42, and 0.17 ppm, respectively. Solutions were prepared in the bottles, and they wereand capped for 2 to 4 h to allow for headspace equilibration prior to sample exposure of indicators. Data was collected for a minimum of five minutes prior to target exposure to establish a baseline for the indicators. For this measurement, bottles containing 30 mL of water were used as the control solutions. Exposure was initiated by exchanging the water containing bottles for those containing target. An alternatively approach accomplished, target exposure was completed by placing the indicator supports over empty Petri dishes for pre- and post-exposure measurements. The exposure measurements were completed by placing the holder over a Petri dish (60 mm; total volume 57 mL) containing 1 mL of warmed target solution (. Prior to exposure under these conditions, the dish was warmed for 10 min in oven at 60°C oven).

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Results and Discussion. The titanyl indicators were evaluated using the prototype reflectance sensor with 30 mL solutions of target (30 mL) in 200 mL Nalgene bottles. This experiment is significantly different from those described in the original report on these materials.[13] Initial characterization utilized a sealed 9 L headspace over 1 L of target solution with a fan generating impacting air flow. Under these conditions, the target content at even the lowest utilized concentrations (0.1 ppm vapor) would not change over the course of the measurement. The experiments of the current study were not conducted in a sealed headspace nor did they utilize a large target excess; concentrations of target were expected to change over time. Ambient temperatures were between 24 and 27°C with relative humidity between 43 and 55%. Though the prototype device reports red, green, and blue (RGB) color values (Figure 2, Panel A), changes observed for the titanyl indicators were much more dramatic for the blue channel than for the red or green. The Wedata hasve been normalized the datausing the average value from the pre-exposure measurement to (Figure 2, Panel B) based on the average value of the pre-exposure measurement, to account for sensor to sensor variation in the data. In Figure 2 (Panel B), we, and report the changes in the blue channel over time following exposure of indicators to various concentrations of hydrogen peroxide. The rates of change in reflectance for the titanyl indicators (blue and green values only) were found to be concentration dependent with saturation of the indicator occurring at ~9.5 h for the 3% target solution and ~15.5 h for the 1.2% target solution.

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Measurements were continued to 66.5 h; with none of the lower other target concentrations resulteding in indicator saturation. (Additional results provided in the Supplementary Material) Previous work with the prototype sensor utilized porphyrin indicators and focused on chemosorptive interactions,[14, 16] often of a reversible nature. The titanyl compounds utilize a reactive interaction (non-reversible) with the peroxide vapor. Representative porphyrin indicators were evaluated for comparison to the titanyl materials (Figure 3). The CoDIX indicator showed significant changes in reflectance upon exposure to hydrogen peroxide while changes in CuDIX reflectance were smaller and NiDIX did not respond. The changes in CoDIX and CuDIX were found to be reversible when target was removed from the indicator environment. Porphyrin indicators were also found to respond to the presence of sulfuric, nitric, and hydrochloric acid (additional results provided in the Supplementary Material). As previously reported, these DIX variants are also sensitive to alcohol vapors.[14] The reflectance of the titanyl indicators does not change upon exposure to acids or alcohols; they provide unique indication of the presence of peroxide vapor. This specificity Depending on the desired application this specificity may offer a significant advantage depending on the application, to a colorimetric sensor system especially if peroxide vapor is a primary or high value target. There are other aspects that should be considered when designing a sensor systems. The differences between the porphyrin and titanyl indicators serve to illustrate some of these points. The titanyl indicators are dosimetry type reporters;, that is, the reported signal is an integration of exposure concentrations and durations. These indicators will reach a saturation point after which they are no longer useful for detection of target. The porphyrin indicators, on the other hand, respond to increasing and decreasing concentrations with reversible changes in reflectance. This , providesing the potential for continuous or long term applications but while makesing them unsuitable for reporting on total exposure based on interval interrogation. The titanyl indicators could be regenerated through hydrolysis in basic water followed by reaction with oxalic acid; however, treatment of the paper supported indicator with liquids will result in migration. Regeneration would likely also be more expensive than replacement of the indicator. A further consideration in characterization of indicator materials for sensor applications is quantification of target concentrations. The reactive nature of the titanyl indicators was originally described as providing the potential for determination of concentrations of peroxide vapor on the basis of rates of change in indicator color.[13] This description should be qualified to be specific for situations under which the sampled space is stagnant and of sufficient volume to prevent reactant depletion. The rate equations developed under the original work cannot be applied to the experimental conditions utilized in the current study. Time is also an important consideration for the proposed is type of analysis. These are do not necessarily comprise a realizable set of conditions. Dramatic fluctuations in target concentration would be expected for point sensors in an environmental sensing scenario.[17, 18] Acknowledgements. This research was sponsored by the U.S. Naval Research Laboratory (WU# 69-6594). R. Neblett was supported through an Office of Naval Research (ONR) sponsored summer research internship. Participation of R. Liu was through the US Navy Science and Engineering Apprenticeship Program (SEAP). The views expressed here are those of the authors and do not represent those of the U.S. Navy, the U.S. Department of Defense, or the U.S. Government. Scheme 1. Modification of titanyl indicator by peroxide resulting in yellow color.

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Figure 1. Photograph of the prototype sensor system with fresh (A) and exposed (B) titanyl indicators and (C) the CoDIX indicator.

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Figure 2. Response of paper supported titanyl indicators to hydrogen peroxide. (A) Shown here are the as reported red, green, and blue color values (dark to light) for an indicator swatch before and during exposure to the vapor evolving from a solution of 3% H2O2 in water (dashed line marks beginning of exposure period). (B) The normalized blue color values are reported for indicator swatches before and during exposure to vapor evolving from water (black) and, 3, 1.2, 0.3, 0.15, and 0.06% (dark to light) solutions of H2O2 in water.

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Figure 3. Response of paper porphyrin indicator to hydrogen peroxide. (A) Shown here are the normalized red, green, and blue color values (dark to light) for a CoDIX indicator swatch before and during exposure to the vapor evolving from a solution of 3% H2O2 in water (dashed line marks beginning of exposure period). (B) The normalized blue color values are reported for CoDIX indicator swatches before and during exposure to vapor evolving from water (black), 3, 1.2, 0.3, and 0.15% (dark to light) solutions of H2O2 in water.

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References. [1] R.M. Burks, D.S. Hage, Current trends in the detection of peroxide-based explosives, Analytical and Bioanalytical Chemistry, 395(2009) 301-13. [2] M.E. Germain, M.J. Knapp, Optical explosives detection: from color changes to fluorescence turn-on, Chemical Society Reviews, 38(2009) 2543-55. [3] T. Krawczyk, S. Baj, Review: Advances in the determination of peroxides by optical and spectroscopic methods, Analytical Letters, 47(2014) 2129-47. [4] R. Bailey, D.F. Boltz, Differentical spectrophotometric determination of hydrogen peroxide using 1,10-phenanthroline and bathophenanthroline, Analytical Chemistry, 31(1959) 117-9. [5] C. Matsubara, K. Kudo, T. Kawashita, K. Takamura, Spectrophotometric determination of hydrogen-peroxide with titanium 2((5-bromopyridyl)azo)-5-(N-propyl-Nsulfopropylamino)phenol reagent and its application to the determination of serum glucose using glucose-oxidase, Analytical Chemistry, 57(1985) 1107-9. [6] K. Takamura, C. Matsubara, Versatility of the Titanium(IV) - Porphyrin reagent for determining hydrogen peroxide, Bulletin of the Chemical Society of Japan, 76(2003) 1873-88. [7] G.M. Eisenberg, Colorimetric determination of hydrogen peroxide, Industrial and Engineering Chemistry-Analytical Edition, 15(1943) 327-8. [8] B.D. Patterson, E.A. Macrae, I.B. Ferguson, Estimation of hydrogen-peroxide in plantextracts using titanium (IV), Analytical Biochemistry, 139(1984) 487-92. [9] C. Matsubara, N. Kawamoto, K. Takamura, Oxo 5,10,15,20-tetra(4-pyridyl)porphyrinato titanium(IV) - An ultra-high sensitivity spectrophotometric reagent for hydrogen-peroxide, Analyst, 117(1992) 1781-4. [10] F.I. Bohrer, C.N. Colesniuc, J. Park, I.K. Schuller, A.C. Kummel, W.C. Trogler, Selective detection of vapor phase hydrogen peroxide with phthalocyanine chemiresistors, Journal of the American Chemical Society, 130(2008) 3712-3. [11] R.M. Sellers, Spectrophotometric determination of hydrogen peroxide using potassium Titanium (IV) oxalate, Analyst, 105(1980) 950-4.

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[12] J. Muhlebach, K. Muller, G. Schwarzenbach, The peroxo complexes of titanium, Inorganic Chemistry, 9(1970) 2381-90. [13] M. Xu, B.R. Bunes, L. Zang, Paper-Based Vapor Detection of Hydrogen Peroxide: Colorimetric Sensing with Tunable Interface, Acs Applied Materials & Interfaces, 3(2011) 6427. [14] B.J. Johnson, J.S. Erickson, J. Kim, A.P. Malanoski, I.A. Leska, S.M. Monk, et al., Miniaturized reflectance devices for chemical sensing, Measurement Science & Technology, 25(2014) 095101 (10pp). [15] B.J. Johnson, N.E. Anderson, P.T. Charles, A.P. Malanoski, B.J. Melde, M. Nasir, et al., Porphyrin-embedded silicate materials for detection of hydrocarbon solvents, Sensors, 11(2011) 886-904. [16] L. Feng, C.J. Musto, J.W. Kemling, S.H. Lim, K.S. Suslick, A colorimetric sensor array for identification of toxic gases below permissible exposure limits, Chem Comm, 46(2010) 2037-39. [17] P.R. Best, K.E. Lunney, C.A. Killip, Statistical elements of predicting the impact of a variety of odour sources, Water Sci Technol, 44(2001) 157-64. [18] K. Obenschain, J. Boris, G. Patnaik, Using CT-Analyst to Optimize Sensor Placement, Defense and Security, (2004) 14-20.

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Scheme 1 Click here to download high resolution image

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Figure 1 Click here to download high resolution image

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Figure 2 color Click here to download high resolution image

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Figure 2 gray scale Click here to download high resolution image

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Figure 3 color Click here to download high resolution image

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Figure 3 gray scale Click here to download high resolution image

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Supplementary Material Click here to download Supplementary Material: Johnson_SensActB_supporting.pdf

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Author Biographies

Brandy Johnson received a Doctoral degree in Photonics (2004) from Oklahoma State University based on research directed at the application of porphyrins in chemical detection. Since joining the Naval Research Laboratory’s Center for Bio/Molecular Science and Engineering in 2004, she has pursued research into distributed sensing, environmental monitoring, and selfdecontaminating materials.

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Ray Liu is currently a student enrolled at Thomas Jefferson High School for Science and Technology, where he has pursued advanced coursework in chemistry, physics, mathematics, and computer science. He has interned at the Naval Research Laboratory for two consecutive summers and aspires to be an engineer.

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Robert Neblett is a rising senior in the Biology Department at Howard University. He spent the summer of 2015 as an intern at the Naval Research Laboratory. He plans to pursue a graduate degree in biology with his current interests lying at the intersection of science and business.

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Dr. Anthony P. Malanoski received a Doctoral degree (1999) in Chemical Engineering at the University of Massachusetts Amherst, MA. After postdoctoral work at the University of New Mexico and Sandia National Labs in New Mexico, he joined the Naval Research Laboratory (2003). His current research interests include thermodynamic and kinetic reaction modelling of nano-scale systems and metagenomic bioinformatics.

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David Stenger earned his Ph.D. in Biophysics at SUNY at Buffalo in 1989. He is a Senior Scientist in the Center for Bio/Molecular Science and Engineering, Naval Research Laboratory, Washington, DC. His research focuses on distributed sensors, autonomous systems, and advanced diagnostics.

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Jeffrey Erickson earned his PhD degree in Chemical and Biomolecular Engineering at the Johns Hopkins University in 2004. He is currently working as an engineer at the Naval Research Laboratory. His research focuses on incorporating bench-scale research developments into portable devices and bringing them from the laboratory into the field. Miao Xu is currently a postdoctoral fellow in Dr. Ling Zang's group, Department of Materials Science and Engineering, University of Utah. He received his B.S. in chemistry and M.S. in inorganic chemistry from Fudan University and his Ph.D. in Materials Science from University of Utah. His research interests are optoelectronic nanodevices and fluorescent sensors. Ling Zang is a USTAR professor at University of Utah, affiliated with Department of Materials Science and Engineering, Department of Chemistry, and Nano Institute of Utah. He received his B.S. in chemistry from Tsinghua University and Ph.D. in chemistry from the Chinese Academy of Sciences. His current research focuses on nanoscale imaging and molecular probing, organic semiconductors and nanostructures, optoelectronic sensors and nanodevices. Martin Moore received his MS degree in chemistry from Youngstown State University in 1998. He is currently a research chemist at the Naval Research Laboratory focusing on synthesis of small molecules and their analytical characterization.

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