Novel fluorescent security marker. Part II: application of novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitrile nanoparticles in safety paper

Altaf Basta*a, Mauro Missori*b, Adel S. Girgisc, Marco De Spiritod, Massimiliano Papie and Houssni El-Saieda
aCellulose and Paper Dept., National Research Centre, Dokki-12622, Cairo, Egypt. E-mail: altaf_basta@yahoo.com; Fax: +20-23-3371719; Tel: +20-12-22174239
bIstituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, Via Salaria Km 29.300, 00016 Monterotondo Scalo (Rome), Italy. E-mail: mauro.missori@isc.cnr.it; Fax: +39-06-49934168; Tel: +39-06-49934166
cChem. Pesticide Dept., National Research Centre, Dokki-12622, Cairo, Egypt
dIstituto di Fisica, Università Cattolica del Sacro Cuore, Largo F. Vito 1, 00168, Rome, Italy
eFacoltà di Medicina e Chirurgia, Università Cattolica del Sacro Cuore, Largo F. Vito 1, 00168, Rome, Italy

Received 8th August 2014 , Accepted 27th October 2014

First published on 27th October 2014


Abstract

Application of newly investigated fluorescent nanoparticles of 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles as security markers for enhancing the safety property of bagasse-based paper sheets (as valuable documents) was studied. The role of sonication conditions and the type of molecule (morpholinyl or piperidinyl) contained in the heterocyclic compound on production of nanoparticles and their fluorescent behavior were evaluated. The quality of the obtained treated paper was evaluated from examining the fluorescence behavior of the particles-treated paper sheets, in comparison with their fluorescence behavior in water suspension. The success of these investigated fluorescent particles as a security marker was recommended from examining the strength properties of the treated paper and the interaction between the fluorescent particles and the cellulose fibers, via FT-IR-spectra and thermal stability tests, and moreover, from the unfalsifiable safety of the treated documents by erasure technique (chemical and mechanical).


1. Introduction

Paper represents a major product of lignocellulosic materials (agro and wood). Special papers are used for specific purposes such as: waterproof paper, carbon paper, cast-coated paper, durable documents, decorative papers, electrical and magnetic paper, etc. There is a large body of literature concerned with the role of cellulosic fibers, sizing agents, metal complexes, fire retardant additives, coated biopolymers, and the surrounding environment on the quality and durability of paper.1–9 Safety or functional papers are a very special grade of paper having a surface design, hidden warning indicia, or both so as to make obvious any attempt of fraudulent alteration of writing thereon by ink eradicators, mechanical erasure, etc. Such paper is used for bank checks, tickets, postal money orders or other papers having a negotiable value.10

The majority of safety papers currently available on the market react quickly to such attempts of falsification with ink-erasing pencils. This often causes the appearance of a fluorescent yellow color which is not easily visible to the naked eye and which furthermore proves troublesome for certain uses.11 Several methods and systems to mark and protect from falsification valuable documents made of, or supported on paper, are based on the use of secret fluorescent dyes.12 In this context, 2-alkoxy-3-pyridinecarbonitrile derivatives have been synthesized.12,13 These derivatives exhibit remarkable fluorescence properties enabling their employment on paper sheets prepared from non-wood fibrous material (bagasse pulp) to produce security paper. However, while these secret dyes show intense fluorescence and leave the paper substrate on which they are deposited unaffected, known problems include low resistance to chemical degradation and photodegradation.12 In addition, the optical properties are specific to each compound and cannot be modified. The same authors14 succeeded in synthesizing a variety of pyridine derivatives possessing both the aforementioned functional groups, i.e. amino and alkoxy groups oriented at the o- and o′-positions of the pyridine nucleus and neighboring to nitrile functions. In other words, gathering the whole functional moieties responsible for fluorescence properties in a sole structure system in an attempt to optimize a fluorescent active agent with high quantum yield.

On the other hand, fluorescent nanoparticles of organic molecules are attracting increasing interest for their good fluorescence quantum yield and long stability over time.15–19 The electronic and the optical properties of organic nanoparticles, due to weaker intermolecular forces such as hydrogen bonds, van der Waals and hydrophilic/hydrophobic interactions, can easily be modulated to obtain many unusual optical properties and potentially useful emission.20,21

In particular, when the relative orientation of the transition dipole moments of molecules in nanoparticles results in a head-to-tail arrangement, a so-called J-aggregate is formed.20 The optical properties of this structural packing are characterized by the shift of the excitonic band towards lower energy with respect to the absorption of the monomers (bathochromic shift), and high fluorescence quantum yield.22 The opposite behaviour is found for the parallel arrangement of dipole moments (H-aggregates), with excitonic absorption at higher energy than that of the isolated molecule (hypsochromic shift) and quenching of fluorescence.21

When the organic nanoparticles tend to be unstable in solution, often aggregating or precipitating over the course of few days,22 their use is therefore severely hampered. Therefore advances have been gained by adopting the reprecipitation method. This method consists of injecting a dilute solution of monomers into a poor solvent. Being insoluble, monomers aggregate and form a colloidal suspension of particles, whose size ranges from nanometers to micrometers.23,24 To control particle size, sonication was used during the reprecipitation process.25–27 Sonochemical preparation of nanophase materials relies on acoustic cavitation, which is the formation, the growth and implosive collapse of bubbles in a liquid irradiated with high-intensity ultrasound.28 The extremely high temperatures (∼4000 K in water), pressures (>20 MPa) and cooling rates (>107 K s−1) attained during acoustic cavitation inside of the collapsing bubbles are exploited to initiate chemical reactions such as oxidation, reduction, decomposition and promotion of polymerization.29,30 Although details on the formation of organic nanoparticles under sonication still need further investigation, acoustic cavitations seems to be the most probable driving force providing the energy needed in the formation of small nuclei.25

Converting the previously investigated 2-amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile (AEMP) fluorescent active prepared heterocycles to nanoparticles of sizes ranging from tens to hundreds of nanometers was provided by the combination of reprecipitation and sonication.27 This method provides an effective way to produce fluorescent organic nanoparticles for several applications that rely on strong and stable fluorescence, for example for marking important or valuable documents. The surprising results were that the suspension of nanoparticles was found to be stable for more than two years when stored at 4 °C in darkness. It was also found that a remarkable increase in the fluorescence yield was observed as the nanoparticle size decreases. The above features, together with the striking stability of optical and mechanical properties over the course of months, allow for straightforward applications that rely on strong and stable fluorescence such as marking important documents.

These results persuaded us to pursue the surface application of AEMP on paper sheet for production valuable documents (unfalsifiable safety paper). To the best of our knowledge, this work is the first work about fluorescent organic nanoparticles that can be used as secret markers for paper documents. In parallel experiments, 2-amino-6-ethoxy-4-[4-(1-piperidinyl)phenyl]-3,5-pyridinedicarbonitrile particles (AEPP) were synthesized and surface treated the paper sheets.

The overarching objective of this present work is to assess the possibility of applying newly synthesized fluorescent nanoparticles of 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles as security markers for enhancing the safety properties of bagasse-based paper sheets (as valuable documents). The role of sonication conditions and type of molecule (morpholinyl or piperidinyl) contained in the heterocyclic compounds on the production of nanoparticles of fluorescent compounds, and which fluorescent particles are more efficient for document quality, especially against strength and falsification, were also optimized.

2. Experimental

2.1. Synthesis and characterization of fluorescent heterocyclic particles

Novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles were prepared according to the previously reported procedures,14 and are summarized with their analyses in the following methods and scheme (Scheme 1).
image file: c4ra08388a-s1.tif
Scheme 1 Route of synthesis the novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitrile derivatives.
2.1.1. 2-Amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile (AEMP or 3c). Reaction time 24 h, almost colourless crystals from n-butanol, mp 234–236 °C, yield 40%. IR: νmax./cm−1 3492, 3371 (NH2), 2223, 2211 (C[triple bond, length as m-dash]N), 1617, 1574 (C[double bond, length as m-dash]N, C[double bond, length as m-dash]C). 1H-NMR (CDCl3): δ 1.35 (t, 3H, CH3, J = 7.2 Hz), 3.21 (t, 4H, morpholinyl 2 NCH2, J = 4.8 Hz), 3.80 (t, 4H, morpholinyl 2 OCH2, J = 4.8 Hz), 4.38 (q, 2H, OCH2CH3, J = 7.2 Hz), 5.48 (s, 2H, NH2), 6.94 (d, 2H, arom. H, J = 8.7 Hz), 7.43 (d, 2H, arom. H, J = 8.7 Hz). MS: m/z (%) 349 (100). Anal. for C19H19N5O2 (349.38): calcd, C 65.31, H 5.48, N 20.05; found: C 65.27, H 5.38, N 20.23%.
2.1.2. 2-Amino-6-ethoxy-4-[4-(1-piperidinyl)phenyl]-3,5-pyridinedicarbonitrile (AEPP or 3d). Reaction time 24 h, pale yellow crystals from n-butanol, mp 246–248 °C, yield 46%. IR: νmax./cm−1 3431, 3343, 3234 (NH2), 2212 (C[triple bond, length as m-dash]N), 1640, 1605 (C[double bond, length as m-dash]N, C[double bond, length as m-dash]C). 1H-NMR (CDCl3): δ 1.34 (t, 3H, CH3, J = 7.2 Hz), 1.50–1.64 (m, 6H, piperidinyl 3 CH2), 3.24 (t, 4H, piperidinyl 2 NCH2, J = 5.7 Hz), 4.37 (q, 2H, OCH2CH3, J = 7.2 Hz), 5.46 (s, 2H, NH2), 6.93 (d, 2H, arom. H, J = 8.1 Hz), 7.40 (d, 2H, arom. H, J = 9.0 Hz). MS: m/z (%) 347 (100). Anal. for C20H21N5O (347.41): calcd, C 69.14, H 6.09, N 20.16; found: C 68.91, H 5.90, N 20.38%.

According to the fluorescence behavior data (quantum yield), the synthesized 2-amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile (3c), possesses higher quantum yield (ϕs relative to the fluorescence quantum yield of quinine sulfate, reaching ∼0.8314).


2.1.2.1 Production of the AEMP and AEPP particles. Nano- and microparticles of fluorescent 3c (AEMP) and 3d (AEPP) heterocyclic compounds were obtained by the reprecipitation method under sonication. Particles were prepared with 100 μL of acetone solutions of AEMP and AEPP (1 mM), injected into 10 mL of deionized and 0.2 μm filtered water used as a non-solvent environment23,31 (see Table 1tbl1 for more details).
Table 1 Conditions of preparation of fluorescent particles and their propertiesa
Sample code Fluorescent cpd Solution volume Water volume Sonication power (W) Process time (min) Temperature (°C) Rh〉 ± σ (nm) σ/〈Rh Zeta potential
a Process conditions used to obtain the AEMP and AEPP particles with an indication of the size and size distribution observed (〈Rh〉is the mean hydrodynamic radius, σ/〈Rh〉 is the polydispersity index of the size distribution; zeta potential is the electric potential between nanoparticles which is correlated to the stability of the nanoparticles suspension).
S1 AEMP 3c 100 μL 10 mL 12.5 1 0 479 ± 378 0.79  
S2 AEMP 3c 100 μL 10 mL 12.5 5 0 264 ± 137 0.52 Approx −20 mV
S3 AEMP 3c 100 μL 10 mL 12.5 30 0 97 ± 39 0.40  
S4 AEPP 3d 100 μL 10 mL 12.5 1 0 499 ± 265 0.53  
S5 AEPP 3d 100 μL 10 mL 12.5 5 0 224 ± 110 0.49 Approx −10 mV
S6 AEPP 3d 100 μL 10 mL 12.5 30 0 240 ± 139 0.58  


Sonication was induced by a Vibra-Cell ultrasonic processor (Sonics & Materials, Inc., USA), at 12.5 W power (see Table 1), connected to a titanium horn to radiate the ultrasonic energy to the liquid contained in a beaker. The beaker was kept in a thermostatic bath to maintain the water at 0 °C temperature during the sonication process. The sonication conditions were selected based on the previous study,27 in order to obtain smaller nanoparticles avoiding their degradation. Therefore we used lower power and low solution temperature and sonication times for compounds 3c and 3d.


2.1.2.2 Dynamic light scattering. The size of the resulting sonicated fluorescent heterocyclic particles was assessed using Dynamic Light Scattering (DLS).32 DLS is a well established, nondestructive technique, allowing the sizing of particles in solution with high accuracy.33 Here measurements were carried out with a commercial light-scattering setup Zetasizer Nano ZS (Malvern), which uses Non-Invasive Backscatter (NIBS) optics, with the scattering angle fixed at 173°. This specific setup allows detecting the autocorrelation function of diffusing particles with size ranging from 0.3 nm to 10 μm. DLS data were analyzed by using the regularized Laplace inversion of the intensity autocorrelation function (CONTIN method);34 the CONTIN method has been used to obtain the intensity-weighted hydrodynamic radii distribution of particles.35 The hydrodynamic radius, defined as image file: c4ra08388a-t1.tif, where k is the Boltzmann constant, T is the absolute temperature, D is the diffusion coefficient and η is the water viscosity, represents the radius of a hypothetical hard sphere that diffuses in water with the same speed as the particle under examination.36
2.1.2.3 Optical properties. Optical absorption of fluorescent compound solutions and their particles dispersed in water were measured in the 200–880 nm wavelength (λ) range by using an Ocean Optics fiber spectrophotometer (model USB 2000). The detector is a 2048 pixel linear silicon CCD. The optical setup allows a spectral resolution of 1 nm FWHM, while data were pitched every 0.38 nm. A quartz cuvette with 1 cm optical path length was used to house the water suspensions of particles. All absorption measurements were normalized to a water filled quartz cuvette.

Excitation spectra and photoluminescence emission of AEMP and AEPP heterocyclic fluorescent particles dispersed in water and on paper substrate were carried out by using a Perkin-Elmer LS 45 fluorescence spectrometer. Quartz cuvette with 1 cm optical path length was used to house the samples. The spectral resolution was 10 nm and data were pitched every 0.5 nm.

Front-surface excitation and collection geometries were used for the investigation of photoluminescence spectra of particles on paper substrate.

2.2. Paper sheets treated and tested

The paper sheet composition is 80% bleached kraft bagasse pulp and 20% bleached kraft softwood pulp. Kaolin (around 10%) and alum-rosin are used as filler and sizing materials, respectively. These paper sheets were kindly delivered from Quena Co. for paper production, Upper Egypt. The bagasse-based paper sheets were sprayed with the fluorescent compound particles (3c, 3d), suspended in deionized water, using an automatic atomizer to achieve a high degree of homogeneity distribution over one phase of sheet (1.4–1.82 g m−2, SD ± 0.03–0.608). The samples were dried, and then conditioned at 20 °C and 50% RH.37

Strength properties, e.g., tensile, tear, and burst indices of paper samples were measured before and after treatment, according to standard procedure.38 Surface ultra-violet examination was carried out using a Documenter® 4500, Spectral Analysis System (Projectina, Swiss).

2.2.1 Interaction between fluorescent heterocyclic particles and paper surface. To investigate interactions between paper and sonicated florescent compounds, IR-spectra were carried out by using a FT-IR-6100 Jasco, Japan (using Attenuated Total Reflection; ATR), on samples of paper. A resolution of 4 cm−1 was used in the measurements. The mean hydrogen bond strength (MHBS) was calculated according to Levdic et al.39

The paper samples were subjected to non-isothermal thermogravimetric analysis. Thermogravimetric analyses were performed in a Perkin-Elmer Thermogravimetric Analyzer TGA7. The samples were heated in pure nitrogen (flow rate 50 mL min−1) at 10 °C minute−1, and within the typical temperature range: 35–600 °C, i.e., until no additional weight loss was observed. Measurements were made using calcined alumina as the reference material. Differential thermogravimetric (DTG) peaks were examined for evidencing different behaviours between the samples, and also for understanding how the surface treatments affected the thermal stability of the fibers. The kinetic parameters based on the weight loss data from TG curve analysis were determined according to the equations described elsewhere.40,41

2.3. TG curve analysis

Kinetic studies, based on the weight loss data, were obtained by TG curve analysis. The activation energy has been evaluated according to Coats and Redfern's method.40 For pseudo homogeneous kinetics, the irreversible rate of conversion of the weight fraction of the reactant was expressed by the following equation:
 
image file: c4ra08388a-t2.tif(1)
where α is the fraction of material decomposed at time t, k is the specific rate constant and n is the order of reaction. The temperature dependence of k is expressed by the Arrhenius equation:
 
image file: c4ra08388a-t3.tif(2)
where A is the frequency factor (s−1) and T is the absolute temperature.

For linear heating rate, a, (deg min−1):

 
image file: c4ra08388a-t4.tif(3)

The activation energy, Ea, of thermal decomposition when n = 1, was calculated by using eqn (4).

 
image file: c4ra08388a-t5.tif(4)

When n ≠ 1, eqn (5) was used;

 
image file: c4ra08388a-t6.tif(5)

Plotting the left-hand-side value of the equation {i.e., log[1 − (1 − α)1 − n/T2(1 − n)]} against 1/T using various values of “n” should give a straight line with the most appropriate value of “n”.41 The least squares method was applied for the equation, using values of “n” ranging from 0.0 to 3.0 in increments of 0.5.41 The correlation coefficient (r) and the standard error (SE) were calculated for each value of “n”. The “n” value, which corresponds to the maximum r and minimum SE, is the order of the degradation process. The activation energies and frequency factors were calculated from the slope and intercept, respectively, of the Coats–Redfern equation with the most appropriate value of “n”.

2.4. Unfalsifiable safety examination

For unfalsifiable safety examination, chemicals and erasures were used to study the effectiveness of erasures on the fluorescence behavior of paper sheets that were sprayed with particles of AEMP and AEPP fluorescent compounds subjected to different sonication processes.

In all eradicators, the ballpoint ink (Reynolds)® type, the erasing of ink by chemical agents (e.g., eradicators, commercial alkaline Clorox® (NaOCl), or benzylalcohol), from different examined hand sheet papers done using a piece of cotton moistened with the erasure solution, and the wiping was done several times carefully with ethanol to avoid destruction of paper or removal of fiber from the surface of the paper as much as possible. While, the mechanical eradication was done using erasers. Papers were examined visually by naked eye and by ultraviolet device by Docucenter® 4500 instrument at a wavelength of 365 nm.

3. Results and discussion

3.1. Dynamic light scattering and optical properties of sonicated novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles particles

The use of controlled sonication conditions during the reprecipitation in water has been demonstrated to be a valid method to control both the size and crystallinity of organic nanoparticles.22,25,26 The sonication parameters were chosen to avoid degradation of compounds AEMP and AEPP during the preparation process. The sonication power was kept constant at 12.5 W. Sonication processes result in a mass loss as compared to the mass of the AEMP or AEPP molecules injected into the water. For 12.5 W sonication power, a final mass concentration of particles around 2–3% of the injected value is expected.27 The yield of the particles production process at 12.5 W sonication power could be explained as due to the interplay between the efficiency in the formation of nanoparticles nuclei and degradation of AEMP or AEPP molecules. In addition, water dissociation induced by ultrasonic energy and the formation of short lived radical species could play an important role in the nanoparticle formation process.42

The mean hydrodynamic radii, 〈Rh〉, together with the polydispersity index of the size distribution (σ/〈Rh〉) and the zeta potential are reported in Table 1. Zeta potential is the electric potential between nanoparticles which is correlated to the stability of the nanoparticles suspension.

The mean hydrodynamic radii 〈Rh〉 and the polydispersity index of the AEMP and AEPP particles dispersed in water are shown in Fig. 1.


image file: c4ra08388a-f1.tif
Fig. 1 The mean hydrodynamic radii 〈Rh〉 together with the polydispersity index of the size distribution (σ/〈Rh〉) of 3c (AEMP) and 3d (AEPP) particles dispersed in water.

DLS measurements showed that the sonication time has a profound effect besides the included groups of novel 6-alkoxy-2-amino-3,5-pyridinedicarbonitriles. AEMP and AEPP samples show two distinct behaviors of 〈Rh〉 and σ/〈Rh〉 with sonication time. For the sample containing the morpholinyl group (AEMP; 3c), a maximum value of 〈Rh〉 = 479 nm was observed for 1 minute of sonication. A smaller particle size was obtained with 5 minutes of sonication while extending the sonication time to 30 min resulted in the smallest size of 97 nm. The polydispersity index, instead, decreased from 0.79 to 0.33 and rose slightly to 0.40 with increasing the time from 1 to 5 and finally to 30 min (Table 1 and Fig. 1).

As can be noted, all the average sizes of piperidinyl-containing fluorescent compounds (AEPP; 3d) lie in the hundred nanometres region. Particle sizes are less affected by the sonication time, especially when extending the time from 5 min to 30 min, where the particle sizes were 224 nm, and 240 nm and polydispersity indices were about 0.49 and 0.58, respectively. It could be concluded that substituting the morpholinyl groups with piperidenyl groups in the investigated fluorescent heterocyclic compounds does not provide nanoscale particles, but rather in the hundreds of nanometers scale.

The zeta potential, which indicates the electrical potential between nanoparticles, of the AEPP is two times that of AEMP, indicating the superior stability of the sonicated particles of AEPP fluorescent compound over AEMP.

UV-vis absorption spectra of sonicated AEMP and AEPP particles, in water suspension, are shown in Fig. 2. For AEMP particles in water suspension two maxima in the absorption spectrum (Fig. 2) at about 340 nm (3.65 eV) and 370 nm (3.35 eV), and a tail towards longer wavelength are evident, in agreement with previously published results.27 The spectroscopic features have been assigned to the n–π* and π–π* transitions of the pyridine nuclei of AEMP molecules in nanoparticles, and a charge-transfer (CT) exciting band of the extended crystal structure, as previously observed in the absorption spectra of other organic nanoparticles.15,22 For AEPP particles in water suspension only one shoulder at about 325 nm in the absorption spectrum is evident (Fig. 2).


image file: c4ra08388a-f2.tif
Fig. 2 Ultraviolet-visible absorption spectra of fluorescent heterocyclic sonicated AEMP and AEPP particles in water suspension.

The fluorescence excitation spectra of AEMP and AEPP nanoparticles with 5 minutes of sonication time, displayed in the lower panels of Fig. 3 and 4, show similar features to those of the absorption spectra in Fig. 2.


image file: c4ra08388a-f3.tif
Fig. 3 Excitation and emission bands of AEMP particles in water and on paper substrate.

image file: c4ra08388a-f4.tif
Fig. 4 Excitation and emission bands of AEPP particles in water and on paper substrate.

It is obvious that the synthesized morpholine-containing heterocyclic compounds (AEMP) exhibits two excitation bands with peaks at wavelengths of about 340 nm and 375 nm, while the piperidinyl-containing compound (AEPP) has only one excitation band with a peak at a wavelength of about 335 nm, corresponding to UV absorption maxima (Fig. 2). However, one intense emission band was observed for both compounds at a wavelength of 4[thin space (1/6-em)]002[thin space (1/6-em)]540 nm when exciting at 360 nm. The emission band in the morpholinyl-containing compounds is broader with maximum at higher wavelength (458 nm), and shoulder at 550 nm, than the piperidinyl-containing compounds (only at 432.5 nm). The best fluorescence is obtained when the morpholine (AEMP) ring was introduced comparable with the piperidinyl ring (AEPP).

3.2. Evaluation of surface treated paper sheets

Due to the possible application of the investigated novel fluorescent compounds as security marks of valuable documents, the fluorescence character at moderate sonication time, 5 min, was investigated on paper substrate. We measured the emission spectra and intensity of the emission band in surface treated local bagasse paper sheets, using an excitation wavelength of 360 nm. In this respect, the specific amounts of water suspensions of nanoparticles have been centrifuged and the supernatant solution has been removed to increase the nanoparticle concentration by a factor of 10. The concentrated water suspensions of nanoparticles were sprayed on local bagasse-based paper sheets described in Section 2.2. Their fluorescence properties are illustrated in Fig. 3 and 4, in comparison with the fluorescence of sonicated particles suspended in water.

Fig. 3 and 4 show that the positions of the emission bands of paper treated with morpholinyl-containing fluorescent compounds (S2, AEMP) were shifted to shorter wavelengths (red shift 18.5 nm) compared to that treated with the piperidinyl-containing compound (S5, AEMP); (blue shift 7 nm), both with respect to the emission of fluorescent samples in water. This is ascribed to the probable interaction of the functional groups of the fluorescent compounds with the hydroxyl and carbonyl groups of the paper sheets. This may affects on functional moieties responsible for fluorescence properties (6-alkoxy-2-amino-4-aryl-pyridine functional groups). The presence of a lone-pair electron of oxygen in the morpholinyl ring may promote this interaction effect. This view is in agreement with the assumption reported before that the strong interaction between AEMP and cellulose occurred due to a resonant energy transfer from cellulose to nanoparticles.12

As can be seen, changing the substituted groups is accompanied by changing the position of the emission band. The intensities of the emission bands were nearly the same for paper sheets treated with the two sonicated compounds, S2 and S5, despite the weight gain on the paper of 1.3 g m−2, and 1.7 g m−2, respectively. The relatively lower weight gain of the paper with morpholinyl-containing compound did not affect either the position or the intensity of the emission band, compared with the relatively higher weight gain of the piperidinyl compound (AEPP).

For the mechanical properties, the histograms in Fig. 5 show that treating the paper sheets made from bagasse with the investigated fluorescent compounds, in addition to providing the paper samples with fluorescent character, also improves the quality number, Qz. Quality number (Qz) refers to the trend of all strength properties, i.e., breaking length, burst and tear factors, and number of double folds.41 This observation is reverse to that observed in the case of bagasse pulp-containing paper sheets treated with these unsonicated fluorescent compounds.14 In other words, subjecting the investigated fluorescent compounds to sonication reduced the bad effect of surface treating the local-bagasse paper surface with fluorescent compounds. The extent of improving is not only dependent on the fluorescent compound but also on average particle size. As can be seen, there is a good relation between the decrease in particle size, under the effect of sonication, and the improvement in quality number (Qz). Moreover, treating with the morpholinyl group-containing fluorescent compound (AEMP; 3c) achieves relatively high improvement in the double fold property of paper sheets compared with AEPP (3d). These data confirmed that the presence of the morpholinyl group instead of the piperidinyl group may enhance the interaction of fluorescent compounds with paper fibers and weaken the fiber–fiber hydrogen bonding.


image file: c4ra08388a-f5.tif
Fig. 5 Strength properties of surface treated paper sheets with sonicated morpholinyl containing fluorescence (AEMP) and piperidinyl containing fluorescence (AEPP) compounds.

It is interesting to note that all treated paper sheets are characterized by relatively high strength properties (Qz), compared to untreated local bagasse-based paper sheets.

3.3. Evidence of interaction of sonicated fluorescent compounds with paper samples

The FT-IR data (Fig. 6 and Table 2), emphasizes the view of interaction of functional groups (NH2) of fluorescent macro-and nanoparticles with the functional groups-containing surface of paper sheets, via hydrogen bonds. The mean hydrogen bond strength (MHBS) in the most treated samples (0.934–1.225) is relatively higher than the untreated paper sample (0.917). The presence of oxygen in the hetero-ring, e.g., morpholinyl, or sonication for a relatively lower period restricted the formation of hydrogen bonds, whereas the MHBS of paper treated with AEMP and sonicated for 1 min is 0.862. This negative action is minimized at relatively lower particle size. Also the band corresponding to the OH symmetric and asymmetric stretching vibrations of the untreated paper sample is red shifted higher on treating with S2 (shifted from 3332 cm−1 to 3281 cm−1), while there is no shift in band position on treating with S5. The band corresponding to the bending vibration of OH and NH at ∼1450 cm−1 follows the same trend. The appearance of more than one band corresponding to C–O–C and/or C–N–C in cyclic compounds in the FT-IR spectra of samples S1–S6 confirmed the inclusion of morpholinyl- or piperidinyl-containing compounds (AEMP, AEPP) on the paper samples. For the case of surface treatment with piperidinyl-containing compounds (S4–S6), an increase in the bands at ∼2920 and ∼2860 cm−1 or splitting of the band corresponding to C[double bond, length as m-dash]C and/or C[double bond, length as m-dash]N at 1640–1655 cm−1 is noticed, especially for sample S6, which was sonicated for 30 min, with smaller particle size (240 ± 139 nm), and higher weight gain (1.83 g m−2).
image file: c4ra08388a-f6.tif
Fig. 6 FT-IR spectra of surface treated paper sheets with sonicated morpholinyl-containing (AEMP) and piperidinyl-containing (AEPP) fluorescent compounds.
Table 2 Main IR-absorption bands and measurements of untreated and fluorescent nanoparticles treated paper sheetsa
Paper sample MHBS νOH or νNH (stretching) νCH (stretching) νC[double bond, length as m-dash]O (stretching) amide I and II (1558–1705 cm1) νOH or νNH (bending) νC–O 1000–1280 cm−1 νCH (rocking)
cm−1 E/A1050 cm−1 E cm−1 E cm−1 E cm−1 E cm−1 E
a MHBS: AOH(Str.)/ACH(Str.).
Untreated 0.917 3856.35 0.294 2933 0.522 1730.2 0.231 1466.9 0.293 1279 0.184 927.0 0.275
3752.22 0.295 2857 0.465 1679.1 0.259 1375.3 0.203 1158.4 0.263 879.8 0.992
3335.67 0.479     1533.5 0.2461 1322.4 0.171 1033.1 1.00 741.9 0.157
S1 AEMP 0.887 3745.5 0.259 2930 0.448 1725.4 0.235 1468.9 0.326 1270.3 0.189 887.5 0.348
3659.7 0.283 2860 0.395 1646.3 0.319 1329.1 0.209 1109.3 1.122 841.2 0.159
3322.2 0.398     1560.5 0.307     1111.2 0.543 645.5 0.474
3177.5 0.352             1031.2 1.00 594.4 1.183
S2 AEMP 0.934 3679.9 (sh) 0.252 2925 0.433 1720.6 (s) 0.200 1464.1 0.302 1279.9 0.245 899.1 0.367
3328.9 0.404 2865 0.364 1655.011 0.286 1431.3 0.294 1109.3 (sh) 0.568 779.5 0.276
        1563.4 (sh) 0.239 1370.6 0.265 1036 1.00 659.9 0.630
S3 AEMP 0.9529 3755.1 0.153 2931 0.394 1719.6 0.149 1464.1 0.229 1276.1 0.127 887.5 0.189
3289.4 0.376 2860 0.326 1653.1 0.222 1435.1 0.210 1150.7 0.204 781.4 0.142
        1552.8 0.173 1371.5 0.155 1108.3 0.511 649.3 0.526
            1322.4 0.134 1032.1 1.00    
S4 AEPP 0.9674 3743.5 (sh) 0.394 2931 0.573 1711.9 (s) 0.423 1466.9 0.475 1263.5 0.419 893.3 0.502
3613.4 0.481 2860 0.526 1658.9 0.46 1342.6 0.440 1161.3 0.474 841.2 0.382
3299.9 0.554     1540.3 (sh) 0.449     1111.2 0.685 755.4 0.497
                1032.1 1.00 650.3 0.754
S5 AEPP 0.989 3753.2 (sh) 0.1200 2929 0.440 1718.7 0.129 1461.2 0.221 1276.1 0.158 900.0 0.223
3335.7 0.435 2867 0.345 1653.1 0.207 1435.2 0.222 1159.41105 0.243 792.9 0.083
            1368.6 0.200 1032.1 0.543 642.5 0.178
            1322.3 0.188   1.0   0.189
S6 AEPP 1.225 3751.3 0.140 2860 0.368 1718.7 (s) 0.133 1461.2 0.223 1289.6 0.124 887.5 0.219
3335.7 (sh) 0.429     1663.7 0.189 1368.6 0.182 1163.3 0.216 841.2 0.068
3229.7 0.451             1104.4 0.550 645.5 0.450
                1034.0 1.00 594.4  


For all paper samples surface treated with AEPP, the relative absorbances of bands related to C[double bond, length as m-dash]O or C[double bond, length as m-dash]N decreased, and the band corresponding to OH is shifted to lower wavelength with increasing MHBS. The foregoing results concluded that subjecting the fluorescent compounds, especially those containing morpholinyl, to sonication leads to motivating their application as a security marker for high strength quality paper sheets.

Analysis of the TGA curve also can be helpful in studying the thermal stability property which is provided to local bagasse-based sheets by surface treatment with the investigated fluorescent compounds; especially with morpholinyl-containing compounds (Table 3 and Fig. 7 and 8). The TGA curves exhibit similar decomposition stages to the untreated paper samples, with differences only in onset temperatures and maximum peak temperatures. Thermogravimetric (TGA and DTGA) degradation curves for untreated paper sheets are represented by Fig. 7, and three stages can be observed. The first stage, from 50–102 °C represents the evolution of residually absorbed water that corresponds to 5.5% of the total weight. The second starts at 209.2 °C and ends at 343.1 °C, with peak maximum at 289.49 °C and occurs due to the depolymerization of paper components (cellulose and hemicellulose). This stage represents a prominent thermal degradation of weight loss of the cellulose to levoglucosan, with weight loss ∼76.5%, and is known as the volatilization stage. The third stage occurs in the region of 380.6–470 °C and has a peak maximum at 438.8 °C, and occurs due to rapid volatilization accompanied by the formation of carbonaceous residue, with weight loss of 96.8% (Table 3). For surface-treated paper samples, the onset temperatures are 210–241 °C and peak temperatures are 296–307 °C. Also, their activation energies (Ea) for the main depolymerization stages are increased to 148–162 kJ mol−1, while Ea for untreated paper is 131 kJ mol−1. It is surprising to notice that the surfaces treated with these investigated compounds retard the degradation, leading to a reduction in the weight loss observed in the volatilization stage to about 75–70.5%, and the final degradation stages to about 95.2–90%. This means that the fluorescent heterocyclic compounds in addition to providing thermal stability to paper sheets also exhibit to some extent fire resistance in the produced treated paper. This view based on the main theory of flame retardance, is to minimize the formation of levoglucosan by lowering the decomposition temperature of cellulose and enhancing char formation by catalyzing the dehydration and decomposition reactions.43–45

Table 3 Kinetic parameters of un- and fluorescent treated bagasse-based paper sheets
Paper sample Stage Temp. range °C DTG peak temp. °C n r Se Ea kJ mol−1 Wt (final)
Untreated 1st 50–102.41 94.563
2nd 209.19–343.11 289.49 1.5 0.989 0.17 131 23.485
3rd 380.6–469.96 438.8         3.16
S1 AEMP 1st 50–125.09 93.555
2nd 223.39–345.96 307.68 1.5 0.986 0.14 143.32 29.498
3rd 381.87–477.01 424.46         9.797
S2 AEMP 1st 50–99.44   93.374
2nd 226.33–344.4 302.2 1.5 0.991 0.15 138.38 25.403
3rd 392.03–487.14 452.51         5.833
S3 AEMP 1st 50–109.37   93.355
2nd 233.46–356.07 305.68 1.5 0.984 0.19 162.07 23.94
3rd 392.1–491.41 455.6         6.351
S4 AEPP 1st 50–137.24   91.686
2nd 210.1–346.22 303.81 1.5 0.946 0.22 131.15 24.984
3rd 389.1–478.5 455.26         6.781
S5 AEPP 1st 50–102.29   92.866
2nd 232.04–344.8 296.52 1.5 0.983 0.19 148.08 27.945
3rd 392.87–516.04 459.85         7.846
S6 AEPP 1st 50–115.3   91.846
2nd 240.78–360.49 304.84 1.5 0.989 0.16 152.13 22.02
3rd 386.3–498.7 452.6         4.751



image file: c4ra08388a-f7.tif
Fig. 7 TGA and DTGA curves of surface-treated paper sheets with sonicated morpholinyl-containing fluorescent compound (AEMP).

image file: c4ra08388a-f8.tif
Fig. 8 TGA and DTGA curves of surface-treated paper sheets with sonicated piperidinyl-containing fluorescent compound (AEPP).

3.4. Chemical and mechanical erasures

Chemical and mechanical erasures were used to study their effectiveness on the safety of surface treated bagasse-based paper sheets with the two candidate sonicated fluorescent heterocyclic compounds. Ballpoint ink (Reynolds®) was used for writing on the different treated hand-sheet paper. In this study, two types of chemical agents were used as eradicators, i.e., commercial alkaline Clorox® (NaOCl), and benzyl alcohol in addition to mechanical erasure. For erasing to be made as effective as possible, the area of the erasure was washed with ethanol (>85%) to extract any residual colored ink material.

From Fig. 9 and 10, it was noticed that sodium hypochlorite erasure had a great effect on erasing ink from both the morpholinyl-containing fluorescent compound (AEMP) and the piperidinyl-containing fluorescent compound (AEPP) treated paper sheets. This agent acts as a reducing agent for the ink color and the erased area appears as a dark stain by ultraviolet light as it destroys the fluorescence behavior of the fluorescent compounds in this area.


image file: c4ra08388a-f9.tif
Fig. 9 Chemical and mechanical erasure of surface-treated paper sheets with morpholinyl-containing fluorescent compound (AEMP).

image file: c4ra08388a-f10.tif
Fig. 10 Chemical and mechanical erasure of surface-treated paper sheets with piperidinyl-containing fluorescent compound (AEPP).

For the case of benzyl alcohol followed by ethanol, when applied on ballpoint ink strokes that were written on treated paper sheets, it was difficult to erase the ink color completely, so the erased area appears under ultraviolet light.

On comparing with untreated paper sheets, it was noticed that sodium hypochlorite or benzyl alcohol when applied for erasing ink color cannot be detected easily and gave good erased sites without changing the color of the untreated paper itself. Time requires for erasing the ink color from the paper increased in the case of paper treated with AEMP than AEPP. This view emphasized the highest surface bonding capacity of sonicated AEMP with relatively lower particle size (S3, 〈Rh〉 = 97 nm). As can be noticed, erasure by benzyl chloride was less effective for removing the ink stroke letters from treated paper produced from different sonicated fluorescent particles, and left a zone that appeared under ultraviolet light. For AEPP-treated paper sheets this zone appeared only for fluorescent particles resulting from sonication for 5 and 30 min.

For mechanical erasure (Fig. 9 and 10), it was noticed that trials required for erasing the ink stroke from the treated paper sheets increased with the treated fluorescent particles resulted from sonication times of 5 min and 30 min. Moreover, the mechanical erasure was less effective for removing the written letters than benzyl chloride.

It was found that the shaved area exhibited more intense fluorescence than the neighboring non-shaved area. So we can conclude that fluorescent compounds, especially with lower particle size, are penetrated inside paper sheets to a greater depth than the ink stroke itself.

4. Conclusion

Fluorescent heterocyclic compounds were applied to provide safety behavior to treated paper sheets. The nanoparticles promise significant improvement to their field of application, therefore in this study we subjected the synthesized 2-amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile (AEMP) and 2-amino-6-ethoxy-4-[4-(4-pipredinyl)phenyl]-3,5-pyridinedicarbonitrile (AEPP) to sonication, as a trial to convert these fluorescent compounds to particles (AEMP and AEPP, respectively). Further application as a security marker for enhancing the safety property of bagasse paper sheets (valuable documents) was assessed.

The application of the resulted particle dispersions of AEMP and AEPP in water to bagasse-based paper sheets provided excellent results. For strength properties, the results showed that the extent of improving is not only dependent on the fluorescent compound but also on the average particle size. There was a good relation between the decrease in particle size, under the effect of sonication, and the improvement in quality number (Qz). Moreover treating with the morpholinyl group-containing fluorescent compound (AEMP) achieved relatively high improvement in the double fold property of paper sheets, compared with AEPP.

It is surprising to notice that these investigated compounds, besides providing thermal stability to the treated paper sheets, also retarded the degradation, and decreased the formation of levoglucosan, and consequently the weight loss. The weight loss of the volatilization stage is about 75–70.5%, and the final degradation stages are about 95.2–90%. This means that the fluorescent heterocyclic compounds, in addition to providing thermal stability to the paper sheets, also provide, to some extent, fire resistant properties to the produced treated paper.

The unfalsifiable safety property of the treated documents by erasure techniques showed that fluorescent compounds, especially with lower particle size, penetrated inside the paper sheets which makes it difficult for them to be falsified.

Acknowledgements

This research work was supported through the Executive Program of Scientific and Technological Cooperation between the Egyptian Ministry for Scientific Research and the Italian Ministry of Foreign Affairs.

References

  1. ANSI/NISO Z39.48-1992. Permanence of Paper for Publications and Documents in Libraries and Archives Published by NISO Press United States of America, R2002.
  2. A. H. Basta, The Role of Chitosan in Improving the Ageing Resistance of Rosin-alum Sized Paper Sheets, Resturator, 2003, 24, 106–117 CAS.
  3. A. H. Basta, Performance of Improved Polyvinyl Alcohol as Ageing Resistance Agent of Rosin Sized Paper and in Restoration Purpose, Resturator, 2004, 25, 129–140 CAS.
  4. A. H. Basta and H. El-Saied, New approach for utilization of cellulose derivatives metal complexes in preparation of durable and permanent colored papers, Carbohydr. Polym., 2008, 74, 301–308 CrossRef CAS PubMed.
  5. T. Lojewski, P. Miskowiec, M. Missori, A. Lubanska, L. M. Proniewicz and J. Lojewska, FTIR and UV-vis as methods for evaluation of oxidative degradation of model paper: DFT approach for carbonyl vibrations, Carbohydr. Polym., 2010, 82, 370–375 CrossRef CAS PubMed.
  6. J. Malesic, J. Kolar and M. Strlic, Effect of pH and carbonyls on the degradation of alkaline paper: factors affecting ageing of alkaline paper, Restaurator, 2002, 23, 145–153 CrossRef CAS.
  7. A. H. Basta and H. El-Saied, Performance of improved bacterial cellulose application in the production of functional paper, J. Appl. Microbiol., 2009, 107(6), 2098–2107 CrossRef CAS PubMed.
  8. A. Mosca Conte, O. Pulci, A. Knapik, J. Bagniuk, R. Del Sole, J. Lojewska and M. Missori, Role of Cellulose Oxidation in the Yellowing of Ancient Paper, Phys. Rev., 2012, 108, 158301 CAS.
  9. C. Corsaro, D. Mallamace, J. Lojewska, F. Mallamace, L. Pietronero and M. Missori, Molecular degradation of ancient documents revealed by 1H HR-MAS NMR spectroscopy, Sci. Rep., 2013, 3, 2896 Search PubMed.
  10. R. L. Brunelle, R. W. Reed, Forensic Examination of ink and paper, Charles, Thomas, Springfreld, IL, 1984 Search PubMed.
  11. A. Honnorat, L. Raux, C. RiouUnfalsifiable safety paper, US Pat. no. 4,725,497, 1988.
  12. A. H. Basta, A. S. Girgis and H. El-Saied, Fluorescence behavior of new 3-pyridinecarbonitrile containing compounds and their application in security paper, Dyes Pigm., 2002, 54, 1–10 CrossRef CAS.
  13. N. Mishriky, F. M. Asaad, A. S. Girgis, Y. A. Ibrahim, New pyridine carbonitriles from fluoro arylpropenones, Recueil des Travaux Chimiques des Pays-Bas, 1994, 113, pp. 35–39 Search PubMed.
  14. A. H. Basta, A. S. Girgis, H. El-Saied and M. A. Mohamed, Synthesis of fluorescence active pyridinedicarbonitriles and studying their application in functional paper, Mater. Lett., 2011, 65, 1713–1718 CrossRef CAS PubMed.
  15. H.-B. Fu and J.-N. Yao, Size Effects on the Optical Properties of Organic Nanoparticles, J. Am. Chem. Soc., 2001, 123, 1434–1439 CrossRef CAS.
  16. D. Xiao, L. Xi, W. Yang, H. Fu, Z. Shuai, Y. Fang and J. Yao, Size-Tunable Emission from 1,3-Diphenyl-5-(2-anthryl)-2-pyrazoline Nanoparticles, J. Am. Chem. Soc., 2003, 125, 6740–6745 CrossRef CAS PubMed.
  17. H. Y. Kim, T. G. Bjorklund, S.-H. Lim and C. J. Bardeen, Spectroscopic and Photocatalytic Properties of Organic Tetracene Nanoparticles in Aqueous Solution, Langmuir, 2003, 19, 3941–3946 CrossRef CAS.
  18. M. Abyan, L. Bîrla, F. Bertorelle and S. Fery-Forgues, Morphology control of organic luminescent microcrystals and approach of their optical properties, Comptes Rendus Chimie Matériaux moléculaires, 2005, 8, 1276–1281 CrossRef CAS PubMed.
  19. S. Fery-Forgues, M. Abyan and J.-F. Lamere, Nano- and Microparticles of Organic Fluorescent Dyes, Ann. N. Y. Acad. Sci., 2008, 1130, 272–279 CrossRef CAS PubMed.
  20. C. Destrée, S. George, B. Champagne, M. Guillaume, J. Ghijsen and J. B. Nagy, J-complexes of retinol formed within the nanoparticles prepared from microemulsions, Colloid Polym. Sci., 2008, 286, 15–30 Search PubMed.
  21. Q. Fang, F. Wang, H. Zhao, X. Liu, R. Tu, D. Wang and Z. Zhang, Strongly Coupled Excitonic States in H-Aggregated Single Crystalline Nanoparticles of 2,5-bis(4-methoxybenzylidene) Cyclopentanone, J. Phys. Chem. B, 2008, 112, 2837–2841 CrossRef CAS PubMed.
  22. R. O. Al-Kaysi, A. M. Mueller, T.-S. Ahn, S. Lee and C. Bardeen, J Effects of Sonication on the Size and Crystallinity of Stable Zwitterionic Organic Nanoparticles Formed by Reprecipitation in Water, Langmuir, 2005, 21, 7990–7994 CrossRef CAS PubMed.
  23. H. Kasai, S. H. Nalwa, H. Oikawa, S. Okada, H. Matsuda, N. Minami, A. Kakuta, K. Ono, A. Mukoh and H. A. Nakanishi, Novel Preparation Method of Organic Microcrystals, Jpn. J. Appl. Phys., 1992, 31, L1132–L1134 CrossRef CAS.
  24. H. Kasai, H. Kamatani, S. Okada, H. Oikawa, H. Matsuda and H. Nakanishi, Size-Dependent Colors and Luminescences of Organic Microcrystals, Jpn. J. Appl. Phys., 1996, 35, L221–L223 CrossRef CAS.
  25. P. Kang, C. Chen, L. Hao, C. Zhu, Y. Hu and Z. Chen, A novel sonication route to prepare anthracene nanoparticles, Mater. Res. Bull., 2004, 39, 545–551 CrossRef CAS PubMed.
  26. A. Perepogu and P. Bangal, Preparation and characterization of free-standing pure porphyrin nanoparticles, J. Chem. Sci., 2008, 120, 485–491 CrossRef CAS PubMed.
  27. M. Missori, M. De Spirito, L. Ferrari, S. Selci, A. Gnoli, G. Arcovito, A. S. Girgis, H. El-Saied and A. H. Basta, Preparation and optical properties of 2-Amino-6-ethoxy-4-[4-(4-morpholinyl)phenyl]-3,5-pyridinedicarbonitrile nanoparticles: a security marker for paper documents, J. Nanopart. Res., 2012, 14, 649–651 CrossRef.
  28. W. B. McNamara, Y. T. Didenko and K. S. Suslick, Sonoluminescence temperatures during multi-bubble cavitation, Nature, 1999, 401, 772–775 CrossRef CAS PubMed.
  29. K. S. Suslick, Sonochemistry, Science, 1990, 247, 1439–1445 CAS.
  30. B. M. Teo, F. Grieser and M. Ashokkumar, High Intensity Ultrasound Initiated Polymerization of Butyl Methacrylate in Mini-and Microemulsions, Macromolecules, 2009, 42, 4479–4483 CrossRef CAS.
  31. F. Bertorelle, D. Lavabre and S. Fery-Forgues, Dendrimer-Tuned Formation of Luminescent Organic Microcrystals, J. Am. Chem. Soc., 2003, 125(20), 6244–6253 CrossRef CAS PubMed.
  32. F. Andreasi Bassi, G. Arcovito, M. De Spirito, A. Mordente and G. E. Martorana, Self-similarity properties of alpha-crystallin supramolecular aggregates, Biophys. J., 1995, 69, 2720–2727 CrossRef CAS.
  33. M. De Spirito, G. Arcovito, M. Papi, M. Rocco and F. Ferri, Small-and wide-angle elastic light scattering study of fibrin structure, J. Appl. Crystallogr., 2003, 36, 636–641 CrossRef CAS.
  34. G. Maulucci, M. De Spirito, G. Arcovito, F. Boffi, A. C. Castellano and G. Briganti, Particle size distribution in DMPC vesicles solutions undergoing different sonication times, Biophys. J., 2005, 88, 3545–3550 CrossRef CAS PubMed.
  35. S. Provencher, CONTIN: a general purpose constrained regularization program for inverting noisy linear algebraic and integral equations, Comput. Phys. Commun., 1982, 27, 229–242 CrossRef.
  36. C. Mazzuca, B. Orioni, M. Coletta, F. Formaggio, C. Toniolo, G. Maulucci, M. De Spirito, B. Pispisa, M. Venanzi and L. Stella, Fluctuations and the rate-limiting step of peptide-induced membrane leakage, Biophys. J., 2010, 99, 1791–1800 CrossRef CAS PubMed.
  37. International Standard, ISO 187, 1990 (E).
  38. The institution of paper chemistry, Appleton, Wisconsin (1952), institute method no. 411–5 Oct. 1st (1951).
  39. I. Levdik; M. D. Inshakov; E. P. Misyurova; V. N. Nikitin. Study of pulp structure by infra-red spectroscopy, Tr. Vses Nauch. Issled. Irst. Tsellyul Bum. Prom., 1967, 52: 109.
  40. A. W. Coats and J. P. Redfern, Kinetic parameters from thermogravimetric data, Nature., 1964, 201, 68 CrossRef CAS.
  41. A. H. Basta, Preparation, characterization and properties of paper sheets made from chemically modified wood pulp treated with metal salts, Int. J. Polym. Mater., 1998, 42, 1–26 CrossRef CAS.
  42. Q. L. Hong, J. Hardcastle, R. A. J. McKeown, F. Marken and R. G. Compton, The 20 kHz sonochemical degradation of trace cyanide and dye stuffs in aqueous media, New J. Chem., 1999, 23, 845–849 RSC.
  43. P. Serebrenikov, Chemical Methods of Fire. Zbornik Trud, CNILCHI, 1934, 2, 407–409 Search PubMed.
  44. F. Shafizadeh, Y. Z. Lai and C. R. Mclinlyre, Thermal degradation of 6-chlorocellulose and cellulose-zinc chloride mixture. Journal Applied Polymer, Science, 1978, 22, 1183–1193 CAS.
  45. F. Shafizadheh, R. M. Furneauk, T. G. Cochern, J. P. School and Y. S. Sakai, Production of levoglucosan and glucose from pyrolysis of cellulosic materials. Journal Applied Polymer, Science, 1979, 23, 3525–3539 Search PubMed.

This journal is © The Royal Society of Chemistry 2014
Click here to see how this site uses Cookies. View our privacy policy here.