Optimization of single-walled carbon nanotube solubility by noncovalent PEGylation using experimental design methods

Figures & data

Table 1 Experimental runs designed by D-optimal model, using qualitative (phospholipid-polyethylene glycol [Pl-PEG] type) and quantitative variables (single-walled carbon nanotube [SWCNT] aqueous concentration and loading efficiency) on the basis of two responses

Run	D-optimal design variables			Measured responses
	Pl-PEG/SWCNT weight ratio (A)	Sonication time (minutes) (B)	Pl-PEG type (C)	SWCNT aqueous concentration (mg/mL)	Loading efficiency (%)
7	10.00	60.00	2000	0.23	95.31
3	5.00	60.00	5000	0.21	91.57
5	5.00	120.00	2000	0.04	52.62
4	10.00	92.47	5000	0.31	60.93
1	7.89	120.00	5000	0.58	5.31
13	5.47	95.79	5000	0.52	92.33
15	9.09	60.00	5000	0.77	26.53
18	5.00	60.00	2000	0.24	16.20
19	5.00	114.63	5000	0.12	30.01
11	10.00	120.00	2000	0.32	50.77
9	5.00	60.00	2000	0.10	78.99
2	8.75	109.25	2000	0.64	47.92
17	8.13	97.41	5000	0.71	80.90
16	6.88	117.19	2000	0.36	97.83
6	7.89	120.00	5000	0.61	92.20
8	10.00	60.00	2000	0.14	74.78
14	9.09	60.00	5000	0.90	90.16
10	5.00	60.00	2000	0.13	70.16
12	5.00	114.63	5000	0.14	55.60
20	9.09	60.00	5000	0.84	95.00

Note: A, B, and C are notations used for the experimental design studies.

Abbreviation: PI-PEG, phospholipid-polyethylene glycol.

Table 2 Independent variables, variable types, and their constraints used for D-optimal design and optimization of functionalization process of single-walled carbon nanotubes (SWCNTs)

Factorsa	Factor type	Lower limit	Upper limit	Goal
Pl-PEG/SWCNT weight ratio (A)	Nominal, independent	5	10	In range
Sonication time (minute) (B)	Nominal, independent	60	120	In range
Pl-PEG type (C)	Categorical, independent	Pl-PEG 2000-NH2	Pl-PEG 5000-NH2	In range
SWCNT aqueous concentration (mg/mL)	Dependent	0.68	0.85	Maximize
Loading efficiency	Dependent	75	95	Maximize

Note:

a A, B, and C are notations used for the experimental design studies.

Abbreviation: PI-PEG, phospholipid-polyethylene glycol.

Figure 1 Ultraviolet-visible spectra of A) pure single-walled carbon nanotubes (SWCNTs), B) phospholipid-polyethylene glycol 5000-SWCNTs, and C) P7-SWCNTs.

Figure 2 Ultraviolet-visible calibration curve constructed with various concentrations of P7-single-walled carbon nanotubes (SWCNTs) and used for the determination of the aqueous solubility of phospholipid-polyethylene glycol-functionalized SWCNTs.

Figure 3 Ultraviolet calibration curve generated with known concentrations of both amine-containing compounds and used for the measurement of free amine content.

Figure 4 Contour plots of single-walled carbon nanotubes (SWCNTs) functionalized with phospholipid-polyethylene glycol (PI-PEG) 2000-NH₂ for A) aqueous concentration and B) loading efficiency.

Figure 5 Contour plots of single-walled carbon nanotubes (SWCNTs) functionalized with phospholipid-polyethylene glycol (PI-PEG) 5000-NH₂ for A) aqueous concentration and B) loading efficiency.

Figure 6 Raman spectra of A) pure single-walled carbon nanotubes (SWCNTs), B) phospholipid-polyethylene glycol (Pl-PEG) 5000-SWCNTs, and C) Pl-PEG 2000-SWCNTs.

Table 3 Optimum solutions and the corresponding responses proposed on the basis of optimum responsesa

Solution	PEG/SWCNT weight ratio (A)	Sonication time (minute) (B)	SWCNT aqueous concentration (mg/mL)	Loading efficiency (%)
1	7.79	60	0.84	98.11
2	7.86	60	0.84	97.35
3	7.68	60	0.84	97.32
4	7.34	60	0.84	97.54

Note:

a Pl-PEG used is Pl-PEG 5000-NH₂. Desirability equals 1.

Abbreviations: PI-PEG, phospholipid-polyethylene glycol; SWCNT, single-walled carbon nanotube.

Figure 7 Contour plot of optimized samples constructed with phospholipid-polyethylene glycol (Pl-PEG) 5000-NH₂, displaying optimum results for both responses, simultaneously.

Abbreviation: SWCNT, single-walled carbon nanotube.

Figure 8 Thermogravimetric analysis (TGA) graphs of A) nonfunctionalized pure single-walled carbon nanotubes (SWCNTs) and B) phospholipid-polyethylene glycol 5000-SWCNTs.

Table 4 Validation of optimization process

Sample	PEG/SWCNT weight ratio	Sonication time (minute)	Pl-PEG type	SWCNT aqueous concentration (mg/mL)		Loading efficiency (%)		Eda
				Pib	Oic	Pi	Oi
1	7.79	60	Pl-PEG 5000	0.84	0.81	98.11	92.70	5.41
2	7.86	60	Pl-PEG 5000	0.84	0.78	97.35	96.00	1.35
3	7.68	60	Pl-PEG 5000	0.84	0.75	97.32	93.00	4.32
4	7.34	60	Pl-PEG 5000	0.84	0.82	97.54	94.50	3.04

Notes:

a Euclidean distance;

b Predicted value;

c Observed value.

Abbreviations: PI-PEG, phospholipid-polyethylene glycol; SWCNT, single-walled carbon nanotube.

Figure 9 Scanning electron microscopy images of A) nonfunctionalized pure single-walled carbon nanotubes (SWCNTs) and B) phospholipid-polyethylene glycol 5000-SWCNTs.

Figure 10 Homogenous dispersion of functionalized single-walled carbon nanotubes (sample from Table 1, run 14) A) at the time of preparation, B) after storage for 1 month, and C) after storage for 2 months.