The specific reaction between amylose in starch and iodine leads to the formation of a deep blue complex that can be used as a semiquantitative test for iodine. In presence of iodide, water insoluble iodine leads to triiodide anion (Reaction 3.3) which is highly soluble and complexes with amylose causing the deep blue color (Reaction 3.4). Iodine can be produced reacting sodium nitrite with potassium iodide and sulfuric acid (Reaction 3.5).32
I2+ I− → I3− (3.3)
I3−+ starch → blue colored complex (3.4)
2N aN O2+ 2KI + 2H2SO4 → I2+ 2N O + K2SO4+ N a2SO4+ 2H2O (3.5)
As start point to develop sodium nitrite, sulfuric acid, and starch inks, the concentrations recommended by Burriel et al. were used.33 To 0.5 ml of sample,
1 or 2 drops of sulfuric acid 1 M were added until the sample turned acidic, then 210 g/L of soluble starch and one solid crystal of sodium nitrite were added. In order to assure that iodide is the limiting reagent and since all inks should be in liquid state, a very concentrated sodium nitrite solution was prepared: 0.7 g/ml. In the case of the soluble starch solution, a filtration step was carried out to separate amylopectin, which does not participate in the colorimetric reaction and is insoluble in water compromising the stability of the ink. Only an amylose solution was used for ink preparation. 1 M sulfuric acid solution was also prepared. Using these reagents, Reaction 3.5 was tested on paper. Color changes were successfully observed on paper; however as previously mentioned, due to imprecise manual reagent deposition on paper, discrimination of various iodide concentrations was not possible. Afterward, we proceeded to fabricate inks from these reagents.
The three solutions -sodium nitrite, sulfuric acid, and amylose– are aqueous. Sodium nitrite and sulfuric acid solutions present very similar surface tension and viscosity compared to water. However, amylose solution presents a very high vis- cosity due to its high concentration (see Figure 3.2, pink data set). Filtered amy- lose solution was diluted using water in order to decrease the viscosity to make it more closely match the viscosity of water. Shear stress versus shear rate was mea- sured for each dilution and in Figure 3.2 the corresponding rheological curves can be observed. Commercial ink rheological curves are included for comparison. As can be observed, amylose solution has a pseudoplastic behavior. As expected, the more water added to the amylose solution, the lower the viscosity. For 10 %(v/v) amylose solution, the viscosity is approximately 9 mPa·s. We verified that al- though the viscosity of 10 %(v/v) amylose solution and pigmented inks differ in 6.3 mPa·s, adjustment of the viscosity was not required for good inkjet printing. Thus, 10 %(v/v) amylose solution, which corresponds to 21 g/l of filtered soluble starch, was selected.
Next, different experiments were conducted with each of the solutions to de- termine the appropriate composition of additives required for proper inkjet prin- ting. The surface tension of the chemical solutions is around 72 mN/m, approxi- mately three times higher than the surface tension of standard pigmented ink (25.75 mN/m). Triton X-100 and Tween-80 were used as surfactant to reduce the
Figure 3.2: Rheometric curves of the different concentrations (expressed in %(v/v)) of amylose solution at 25◦C. A) Shear stress versus shear rate. B) Viscosity versus shear rate. Commercial ink rheological curves are shown for comparison purposes. surface tension of the chemical solution; the results at different surfactant con- centrations are shown in Table 3.2. 1 %(v/v) Triton X-100 resulted to be the most efficient achieving values between 27–30 mN/m for the different inks and allowing for good jetting performance. Higher concentrations of Triton X-100 did not decrease the surface tension substantially. Regarding viscosity, as previously mentioned, viscosity values similar to water did not need to be adjusted for good inkjet printing.
Table 3.2: Starch, sodium nitrate, and sulfuric acid ink surface tension varying the concentration of two surfactants, Tween-80 and Triton X-100.
Surfactant Surfactant surface tension (mN/m)
conc. Starch ink Sodium nitrite ink Sulfuric acid ink
% (v/v) Tween Triton Tween Triton Tween Triton
0,1 66,53 37,34 53,24 34,72 43,44 33,25
0,5 54,85 30,86 38,93 28,86 38,13 28,45
1 51,08 29,65 37,79 27,35 36,67 27,34
2 48,28 29,49 36,92 27,29 34,52 27,12
When formulating the inkjet printing ink, the compatibility of the ink with the printer components need to be taken into account.30 Thus, in the case of the
sulfuric acid ink, corrosion studies were conducted as acids mays corrode copper in the printer injectors and as a result damage the printer. 1 M solution of sulfuric acid and 4 different concentrations of the corrosion inhibitor benzotriazole were used. 1 M benzotriazole concentration was the minimum concentration of inhibitor required in order to form a passivating benzotriazole self-assembly monolayer that protects the copper from corrosion (Figure 3.3). For 1 M benzotriazole, the mass loss of the copper plate was less than 0.1 % as can be seen in Table 3.3, which was considered negligible.
Figure 3.3: Copper plates incubated for 12 days in a 1 M solution of sulfuric acid with different concentrations of benzotriazole. Benzotriazole concentrations indicated above each copper sample.
Table 3.3: Corrosion inhibition study by 1 M sulfuric acid on copper in presence of different concentrations of benzotriazole.
Benzotriazole concentration [M] Weight loss [%(w/w)]
0 0.33
0.05 0.17
1 0.09
2 0.08
Table 3.4 summarizes the composition of the sodium nitrite, sulfuric acid and starch inks (Lanes 1-3). Once the three chemical inks that participate in the reactions 3.3–3.5 were developed, analytical test strips were produced. Amylose, sodium nitrite, and sulfuric acid inks were printed following that order over the same paper area. Each chemical ink was introduced in a different printer cartridge and by selecting the equivalent colored cartridge it was possible to select which ink was printed at each time. The dipstick was tested through immersion in solutions with different iodide concentration. Results were evaluated after 30 s (Figure 3.4).
Table 3.4: Composition of various chemical inks.
Ink Main component Additives γ (mN/m) µ (mPa·s)
Starch 21 g/L filtered soluble starch 1 % Triton X-100 29.7 1.53 Sodium nitrite 0.7 g/ml N aN O2 in H2O 1 % Triton X-100 27.3 3 Sulfuric acid 1 M H2SO4 in H2O 1 % Triton X-100, 1 M benzotriazole 27.3 0.99 3,3’,5,5-TMB 2.5 mM 3,3’,5,5- TMB in ethanol – 22.1 1.21
References: γ is the surface tension; and µ is the viscosity.
Figure 3.4: Color observed in printed paper with amylose, sodium nitrite and sulfuric acid inks after immersion in potassium iodide solutions of different con- centrations. Iodide concentrations indicated above each corresponding colored paper.
Figure 3.4 shows how color differences allow determining semi-quantitatively a wide range of iodide concentrations demonstrating successful ink preparation and printing. However, while this worked as proof of concept, the level of detection was not sufficiently low to detect physiological levels of iodine in the urine. It should be noted that chemical reagents, chemical inks and printed paper strips were stored at room temperature without observing activity loss or ink instability for several months.