[Android] how can I access picture in sdcard ?

This my code

if (Application.platform == RuntimePlatform.Android) { Debug.Log ("This is android platform");

         DirectoryInfo dirAndroid_left_seq = new DirectoryInfo (Application.persistentDataPath + "/seq_test/lucky_left");

         

         FileInfo[] infoLeftAndroid = dirAndroid_left_seq.GetFiles ("*.png");

         foreach (FileInfo f in infoLeftAndroid) {
             //Debug.Log (f.ToString());

             Texture2D tex = null;
             byte[] fileData;

             if (File.Exists (f.ToString ())) {
                 fileData = File.ReadAllBytes (f.ToString ());
                 tex = new Texture2D (1920, 1080);
                 tex.LoadImage (fileData); //..this will auto-resize the texture dimensions.

                 jpegViewerLeft.sortList.Add (tex);
             }
         }

But It's not working. How can access sdcard with path . thank you in advance.