Modulating photochemical reactions in Langmuir monolayers 

of Escherichia coli lipid extract with the binding mechanisms 

of eosin decyl ester and toluidine blue-O photosensitizers 

 

Lucas G. Moreira1, Alexandre M. Almeida Jr.1, Tyler Nield1,2, Sabrina A. Camacho1,3 

and Pedro H.B. Aoki1 

 

1 School of Sciences, Humanities and Languages, São Paulo State University (UNESP), 

Assis, SP, 19806-900, Brazil 

2 Faculty of Engineering, University of Victoria, Victoria, BC, V8P 5C2, Canada 

3 IFSC, São Carlos Institute of Physics, University of São Paulo (USP), São Carlos, SP 

13566-590, Brazil 

 

 

 

 

 

 

 

 

 

  



 

Fig. S1. Surface pressure (mN/m) versus mean molecular area (Ų) of neat EosDEC and 

TBO photosensitizers. 

 

  



Fig. S2a and b shows how the in-plane elasticity of neat E. coli lipid extract monolayer 

is affected upon EosDEC and TBO incorporation, respectively. The in-plane elasticity, 

or surface compressional modulus (Cs
-1), was calculated from the π-A isotherms of Fig. 

2. In the case of EosDEC, the maximum values of elasticity decreased from 113 to 93 

mN/m and 87 mN/m considering E. coli:EosDEC at 1:10 and 1:5, respectively. The TBO 

incorporation also decreased the monolayer elasticity from 113 to 84 mN/m. These 

modifications occur at 30 mN/m (insets), which is relevant from the biological point of 

view since this surface pressure is believed to correspond to the lateral pressure of cell 

membranes. 

 

Fig. S2. In-plane elasticity (Cs
-1) for neat E. coli lipid extract monolayers and for the PSs 

incorporation with (a) EosDEC (1:5 and 1:10) and (b) TBO (10-5 mol/L). The equation 

𝐶𝑠
−1 = −𝐴 (𝑑𝜋 𝑑𝐴)⁄  was used as function of area (cm²/mg of extract) and the surface 

pressure (insets). 

 

  



Langmuir-Schaefer (LS) films of E. coli lipid extract, EosDEC, E. coli:EosDEC (10:1 

v/v) and E. coli + TBO subphase (10-5 mol/L) were grown at a constant surface pressure 

of 30 mN/m. The deposition of the LS films was monitored every 4 layers up to 60 layers 

using UV-Vis absorption spectroscopy, the results of which are presented in Fig. S3. The 

LS film of E. coli lipid extract exhibits three bands in the near UV range, at 200 nm, 227 

nm and at 364 nm (Figure S3a). Similar absorption bands were identified by Van den 

Berg et al. [5] characterizing E. coli HCP (HCP, named ‘prismane protein’), a gene that 

encodes the hybrid-cluster protein in E. coli. A linear growth is observed following the 

absorbance at 200 nm as the number of deposited layers increases (inset – Fig. S3a; R2 = 

0.9693), suggesting that controlled amounts of material were deposited per layer. 

Fig. S3b displays the absorption spectrum for EosDEC LS film with bands located at 

201, 230, 252, 298, 402, 507 and 543 nm, attributed to electronic transitions of EosDEC 

molecule. Particularly, the bands at 507 and 543 nm, are assigned to dimeric and 

monomeric forms of EosDEC, respectively. A trimeric form appears at 402 nm, due the 

high concentration used for EosDEC (1 mM) [6,7]. The EosDEC dimeric form in 

chloroform solution (Fig. S4a) at 455 nm, displaced to 507 nm in the LS film. The 

monomeric form in chloroform at 479 nm and 514 nm (Fig. S4a) is combined in a single 

band and displaced to 543 nm in the LS film. Such changes indicate the formation of J-

aggregates in the LS film, with the dipoles of molecules in a head-to-tail arrangement 

[7,8]. Other three well defined bands of the chloroform solution located at 244, 285 and 

323 nm are assigned to the π-π* transitions in the aromatic rings [9], which is blue-shifted 

at EosDEC LS film to 201 nm, 230-252 nm (splitted) and 298 nm, respectively. The inset 

exhibits the linear growth of the LS film at 200 nm (R² = 0.9986) and at 543 nm (R² = 

0.9948), indicating similar amounts of material per each deposited layer. 



The monomeric (507 nm) and dimeric (543 nm) absorption bands of the EosDEC LS 

film is also observed in the E. coli:EosDEC mixed LS film (Fig. S3c), confirming the 

deposition of EosDEC. A smaller peak at 396 nm confirms a trimeric form. The π-π* 

transitions from the aromatic rings are located at 198, 259 and 313 nm. The main 

modification is related to the dimeric band at 507 nm, which is slightly lower in the 

absorption spectrum than the EosDEC LS film, suggesting the presence of less 

aggregates. In fact, EosDEC have demonstrated a tendency in form highly packed 

monolayers favored by the π-π interactions between the xanthene rings [10]. With respect 

to the monitored absorbance versus the number of layers, a linear growth was also noted 

at 200 nm (R2 = 0.9490) and at 543 nm (R2 = 0.9759). Such behavior suggests that, 

independently of the monitored absorbance, approximated amounts of material were 

deposited per layer (inset – Fig. S3c).  

The LS film of E. coli on TBO subphase (10-5 mol/L) displays absorption bands at 

243, 276, 518, 579 and 654 nm (Fig. S3d), assigned to the absorption modes of the 

deposited TBO. The UV-Vis absorption spectrum of TBO water solution (Fig. S4b) 

presents maximum absorption at 632 nm and a shoulder at 595 nm assigned to the 

monomeric and dimeric TBO species, respectively [11,12]. The monomer band is red-

shifted to 654 nm while the dimer band is splitted and blue-shifted to 518 nm and 579 nm 

[13]. Such behavior indicates the presence of both J- and H-aggregates in the LS film, 

with dipoles arranged head-to-tail and parallel to each other [8]. In addition, the monitored 

absorbance at 230 nm (R2 = 0.9305) and 579 nm (R2 = 0.9873) presented a linear growth, 

suggesting that similar amounts of material were transferred to the substrate per layer 

(inset – Fig. S3d). 



 

Fig. S3. UV–Vis absorption spectra of LS films deposited on quartz substrates. The film 

growth was monitored every 4 layers up to 60 layers for (a) E. coli lipid extract, (b) neat 

EosDEC, (c) E. coli:EosDEC (10:1 v/v) and (d) E. coli on TBO subphase (10-5 mol/L). 

The insets show the absorbance at a fixed wavelength (±200 nm, 543 nm and 579 nm) for 

each LS layer deposited from the different films. 

 

 
Fig. S4. UV–Vis absorption spectra for the LS film (60 layers) of (a) E. coli:EosDEC 

(10:1 v/v) and (b) E. coli + TBO subphase (10-5 mol/L). The spectra of EosDEC 

chloroform solution and TBO aqueous solution (both at 10–5 mol/L) are given as 

reference. 

 



Upon irradiation, no statistical significance was observed for the E. coli:EosDEC (5:1 

v/v) film (Fig. S5). The highly aggregated state of EosDEC molecules at the monolayer 

interface, which is caused by π-π interactions between the xanthene rings [2], may 

decrease the 1O2 quantum yield owing to self-quenching and may be a reasonable 

explanation for the lower photodynamic efficiency. 

 

Fig. S5. Changes in relative area (A/A0) for nonirradiated (black) and irradiated (red) of 

E. coli:EosDEC (5:1 v/v) mixed monolayer. A0 is the extrapolated area for the E. 

coli:EosDEC isotherm at 30 mN/m. 

  



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