Chemicals and reagents
Sulfite pulp (\(DP_{\mathrm {W}}=1300\)) was received from Lenzing (Lenzing, Austria). Carboxymethyl chitosan (\(DS=1\), degree of deacetylation = 94.2%), chitosan lactate and chitosan acetate were purchased from Heppe Medical Chitosan GmbH (Halle/Saale, Germany). Ethanol 99.5% denatured from Grüssing (Filsum, Germany), chlorosulfuric acid for synthesis and PBS from Merck (Darmstadt, Germany), dimethylformamide (DMF) 99% and sodium periodate \(\ge 99.0\%\) were purchased from Sigma Aldrich (Munich, Germany), ethylene glycol \(\ge 99.5\%\) p.a., sodium hydroxide and sodium acetate trihydrate were obtained from Carl Roth (Karlsruhe, Germany). The fluorescence dye SNARF-4F 5-(and-6)-carboxylic acid (SNARF) was purchased from Invitrogen (Darmstadt, Germany). DMEM was purchased from Lonza (Verviers, Belgium). DMF was dried over molecular sieve with a pore size of 3 Å. All other reagents were used without further purification. All aqueous solutions were prepared using deionized water. Dialysis membranes from Spectra/Por® had a molecular weight cut off of 100 Da to 500 Da and of 3500 Da.
Sulfation of cellulose
Sulfation of cellulose was conducted by two methods referring to Zhang et al. (
2011) with slight modifications. For both variations pretreatment was the following: Cellulose (2.50 g; 0.0154 mol) was soaked in 125 ml DMF at ambient temperature for 24 h. Dialysis was carried out using a standard regenerated cellulose membrane with a molecular weight cut off of 3.5 kDa.
Sulfation of cellulose The reaction agent consisting of DMF (25 ml) and chlorosulfuric acid (0.0462 mol, 3.0 eq.; 0.0693 mol, 4.5 eq.; 0.0924 mol, 6.0 eq.) was prepared while cooling in an ice bath. The sulfating agent was added dropwise to the soaked cellulose at room temperature (RT). After 3 h of vigorous stirring the now clear solution was poured in an ethanolic solution (500 ml), which contained NaOH (14.3 g, 0.358 mol), NaOAc\(\cdot\)3 \({\hbox {H}}_2{\hbox {O}}\) (11.05 g, 0.0812 mol) and \({\hbox {H}}_2{\hbox {O}}\) (26.5 ml) filled up to 500 ml with ethanol (99%). The precipitate had to rest at least 12 h before the supernatant was removed by centrifugation. Afterwards the sediment was washed two times with 4% ethanolic/aqueous (v:v; 1:1) sodium acetate solution (100 ml). The product was dissolved in a minimum of \({\hbox {H}}_2{\hbox {O}}\) and the pH was adjusted to 8.0 with glacial acetic acid. Next, the cellulose sulfate was precipitated and dissolved two more times, then dialyzed and lyophilized.
Acetosulfation The reaction agent consisting of DMF (25 ml), chlorosulfuric acid (0.0462 mol, 3.0 eq.) and acetic anhydride (0.123 mol, 8.0 eq.) was prepared while cooling in an ice bath. The sulfating agent was added dropwise to the soaked cellulose and heated to \(50^{\circ }{\hbox {C}}\), followed by 5 h of intense stirring at \(50^{\circ }{\hbox {C}}\). To quench the reaction the solution was poured in an ethanolic solution (500 ml) which contained NaOH (14.3 g, 0.358 mol), NaOAc\(\cdot\)3 \({\hbox {H}}_2{\hbox {O}}\) (11.05 g, 0.0812 mol) and \({\hbox {H}}_2{\hbox {O}}\) (26.5 ml) filled up to 500 ml with ethanol (99%). The precipitate had to rest at least 12 h before the supernatant was removed by centrifugation. The sediment was washed two times with 4% ethanolic/aqueous (v:v; 1:1) sodium acetate solution (100 ml). The product was then dissolved and stirred for 20 h at ambient temperature in NaOH (50 ml, 1 m/l) for deacetylation, thereafter the pH was adjusted to 8.0 using glacial acetic acid. The cellulose sulfate was precipitated and dissolved two more times, then dialyzed and lyophilized.
Measurements
Fourier-transform (FT) Raman spectra were recorded on a Bruker MultiRam spectrometer with a liquid nitrogen cooled Ge diode as detector. All spectra were recorded over a range from 3500 to \({150}\,{\hbox {cm}}^{-1}\) with a resolution of \(4\,{\hbox {cm}}^{-1}\).
The \(^{13}\)C-NMR spectra of CS in \({\hbox {D}}_2{\hbox {O}}\) were recorded at RT on Bruker Avance III 500 MHz (Bruker, Etlingen, Germany) with a frequency of 125.76 MHz, pulse length of \({12.05}\,\upmu {\hbox {s}}\), acq. time of 0.35 s and a relaxation delay of 3 s. The \(^{1}\)H-NMR spectra of oxidized CS in \({\hbox {D}}_2{\hbox {O}}\) were recorded at RT on the same device with a frequency of 500.13 MHz, pulse length of \({12.1}\,\upmu {\hbox {s}}\), acq. time of 3.3 s and a relaxation delay of 1 s.
Size-exclusion chromatography (SEC) was performed on a SEC-7 (Jasco, Germany) equipped with UV detector (UV-975) and a RI detector (RI-930) under following conditions: columns: Suprema precolumn, Suprema 1000, Suprema 30; column temperature: \(30.00^{\circ }{\hbox {C}}\); eluent: 0.1 mol/l \({\hbox {NaNO}}_3\)/0.05% \({\hbox {NaN}}_3\); flowrate: 1.000 ml/min; standards: pullulan. All samples were filtered through a syringe filter (nylon, pore size \(0.45\,\upmu {\hbox {m}}\)) ahead the injection.
Sulfur content was measured using the sulfur analyzer CS-580 by ELTRA. The degree of substitution (
DS) was calculated with Eq.
1 based on the determined sulfur content.
$$\begin{aligned} DS_{\mathrm {Sulf}}=\frac{162.1 \cdot S(\%)}{3207-102.1 \cdot S(\%)} \end{aligned}$$
(1)
The frequency sweep was conducted using an Ares-G2 (TA Instruments) under following conditions: 15 mm parallel plate; temperature:
\(25^{\circ }{\hbox {C}}\); strain:
\(3.0 \times 10^{-3}\); logarithmic sweep; angular frequency: 0.1 rad/s to 100 rad/s.
Determination of aldehyde content A blank consisting of hydroxylammonium chloride (0.4 mol/l; 20 ml) and deionized water (25 ml) was made and the pH was measured. Dialyzed oCS (60.0 mg) was dissolved in deionized water (25 ml) and the pH was adjusted to 7.0 using sodium hydroxide (0.01 mol/l). After adding hydroxylammonium chloride (0.4 mol/l; 20 ml), the reaction mixture was stirred for a minimum of three hours at RT. The released HCl was titrated with NaOH (0.01 mol/l) until the pH of the blank was reached. The amount of aldehyde given as
\(DS_{\mathrm {Ald}}\) was calculated by the following equation:
$$\begin{aligned} DS_{\mathrm {Ald}}=\frac{6 \cdot M_{\mathrm {C}} \cdot 0.01 \cdot t_{\mathrm {NaOH}} \cdot V_{\mathrm {NaOH}}}{C(\%) \cdot m} \end{aligned}$$
(2)
\(M_{\mathrm {C}}\): molar masse of carbon atom (12.01 g/mol),
\(t_{\mathrm {NaOH}}\): titer of 0.01 mol/l NaOH,
\(V_{\mathrm {NaOH}}\): volume of consumed 0.01 mol/l NaOH solution in ml,
\(C(\%)\): carbon content of sample in %,
m: mass of sample in mg.
Detection of pH The pH was measured by fluorescence microscopy with the pH-sensitive dye SNARF. This method has the advantage to detect a local pH inside the hydrogel. However, the pH range is limited by the dyes sensitivity range from pH 5 to 7 (Schädlich et al.
2014). The dye was incorporated into the oCS (
\({200}\,\upmu {\hbox {g}}/{\hbox {ml}}\)) or CMCh (
\(100\,\upmu {\hbox {g}}/{\hbox {ml}}\)) solution. The pH of the hydrogels prepared in PBS was measured once with the dye mixed in the cellulose solution and the once mixed in the CMCh solution to exclude a bias towards more neutral pH values in CMCh or towards more acidic values in the CS solutions.
\(10\,\upmu {\hbox {l}}\) of the two solutions each were pipetted and stirred in a
\(100\,\upmu {\hbox {l}}\) insert inside a glass vial. The vial was placed under the fluorescence microscope consisting of a light source (PhotoFluorII NIR), a microscope (Leica DM4000B) with Nuance EX fluorescence detector and Nuance Software. An image cube was detected with green (515–560 nm) and red (620–660 nm) excitation light. The two different excitations make it possible to compensate varying intensity and spectral broadening (Schädlich et al.
2014). The measurement was repeated without addition of SNARF to measure the background and auto-fluorescence of the hydrogels. The fluorescence spectrum was extracted from a region of interest in the image cube. The spectrum of the blank sample was subtracted. The ratio of the intensities is calculated with Eq.
3 from (Schädlich et al.
2014).
$$\begin{aligned} ratio=\frac{I^{gr}_{609{\mathrm {nm}}}-I^{red}_{679{\mathrm {nm}}}}{ I^{gr}_{625{\mathrm {nm}}}} \end{aligned}$$
(3)
From the
ratio the pH can be calculated with the help of a calibration curve. The calibration was made with PBS with
\(20\,\upmu {\hbox {g}}/{\hbox {ml}}\) SNARF and variation of pH with NaOH and HCl.
Homogeneity and permeability of the hydrogels The transverse relaxation time distribution was characterized with a low-field benchtop Maran Ultra \(^1\)H-NMR spectrometer (Oxford Instruments, UK) including an air flow temperature control and a 3D imaging unit. Water, CMCh solution and the hydrogels were measured with a CPMG sequence consisting of a single \(90^{\circ }\) radio frequency (rf) pulse followed by a series of \(180^{\circ }\) rf pulses with 20480 Echos, \(2{\tau }=135\,\upmu {\hbox {s}}\) and 25 s recycle delay. The Windows Distributed EXPonential analysis software (WinDXP, Oxford Instruments, UK) was used to calculate \(T_2\) distributions with 256 points in the relaxation time range from \(10\times 10^{-6}\,{\hbox {s}}\) to 20 s.
The diffusion coefficients of the water molecules were measured with pulsed field gradient stimulated echo with gradient pulses of variable length. The diffusion time was 400 ms. The maximal gradient strength of 0.922 T/m was used.